Hi Jami. At one time I began collecting numbers, wherever I could find them, relating to the split junk rig. I didn't get very far, but here is what I have at present: (you can click on it to enlarge)
(As you know, these shelf foot sails have a designed camber (which would be the camber if they were made from tin plate) and useful enough when comparing shelf foot sails with shelf foot sails. However the actual camber of the soft cloth sail will be greater, and if comparing with conventionally cut sails, the camber needs to be measured when the panels are inflated, the way Arne does it.
No-one seems to have done it yet with a SJR panel. I just need to add that disclaimer).
I am very happy with the way Serendipity sails, including to windward, but we don't yet have much in the way of objective comparisons. Emmelene looks to be sailing beautifully in the photo in the current home page "featured boat" and Chris is happy. Amiina (the model for most of the subsequent SJR rigs) has proved successful. The parameters for your sail would seem to be within the "successful" range, so if you are a little unsatisfied, maybe we need to look for other parameters which are not shown here.
The one which comes first to my mind is "balance". I have a feeling that when going to windward the lion's share of the work is being done by the jibs, and the wash from the jibs may be (not sure) may be slightly degrading the performance of the mains panels, because on my boat it is the mains which begin to lose that pressure difference first, when pinching up to windward. It seems to me the important point about the SJR is that to get the best from it, the jibs need to be as large and powerful as possible (the limiting factor being the sail's dynamic balance point). This notion is reinforced, in my mind, by a high performing bermuda-rigged trailer boat I once owned (a Noelex 22) which went to windward like a witch, in any decent sort of breeze, on staysail alone - and with beautiful helm balance too. The stay sail in this case was a 3/4 rig, but with fairly long foot - not quite a genoa, but it was a powerful little staysail and it pulled like a mule. (The hull was superbly proportioned too, of course).
Therefore, it seems important to have as great a jib chord as possible. So, the balance ought not to be too much less than the proven 33%. (And for those who are contemplating a SJR for the first time, not too much more either! Be sure you understand the balance calculation for SJR - it is not quite intuitive, and a mistake here could have consequences as 33% is probably not too far from the limit of what is workable. Sorry Jami, you will know all this this, its just another disclaimer which needs to be put in here.)
As you will know, if you were to increase the jib cord on your sail, this may affect the balance calculation, depending on where the extra cloth is placed. If the extra cloth of a new jib were to be added to the leech (reducing the slot width) then the balance calculation won't be affected. If the extra cloth is added to the luff and the battens extended forward accordingly, then the calculated balance will be increased, and the proven limit, to date, is 33% - though we are all waiting for someone to try 35%, which Slieve has found successful on model swingwing racing boats. If there is room to move, within the envelope of your sail plan, then new jibs and extended battens might be worth the shot. It would be interesting to know what is the current calculated balance of your sail, and the slot width - better still to see a diagram of your sail with all these measurements. A diagram would save a lot of words.
A "thought experiment" suggests to me that if the jibs are made ridiculously small, (say, a balance of 5%) they would merely interfere with the main, and in that case the boat might sail better without them. I am not saying for one moment that your jibs are "ridiculously" small, but they may be a tad too small, and it would indeed be interesting to follow your suggestion, and before the season is over, just for fun, try your boat for sailing without them. I expect a SJR with no jibs would be at least as good as any other cat-rigged sloop, sail area for sail area.
My uninformed suspicion is, maybe the SJR really needs that 33% balance to perform at its best, and maybe if there is only room in the design for 25% balance, or less, then a contiguous sail might be a better choice. And it would not surprise me if you find, with larger jibs chord, that your boat would be more powerful to windward.
Its one of those many things that we won't know until someone tries. We all still have much to learn.
(PS the main panels collapsing first, when coming up into the wind, is normal I think. The two-part sheeting for this type of sail works very well on my boat, not really any extra hassle and perfect for controlling twist).
(PPS for those contemplating SJR for the first time, all this talk about balance is fairly important, because the sheeting forces are quite light on a SJR, and if you go over the limit with balance there is a risk that in a high wind it might not be possible to luff up or to let fly the sail! A runaway junk, helmsman with eyes on stalks, scrambling to the windward side to stack out for dear life while at the same time trying to put the helm down, would be an impressive sight - I wouldn't want it to be me!
There has been a suggestion also, that if the battens start bending, this might exacerbate the problem - so we don't want to be right on the limit with balance.)