Can you say what are the potential risks, should I leave the sail this way?

The yard is already quite stong.

Yes,

The balance is affected a bit, which might indeed explain a bit of the change. However, it can’t be the reason for the drive/heel change, can it?

Yesterday I was sailing in 8-10 m/s winds, with moderate swell. With three and four panels the boat felt to be lacking drive, and suddenly I had the feeling that the top panel was acting mostly as a heeling vector.

To test this I lashed the upmost batten to the yard and hoisted the sail (while understanding that the slingpoint and YHP were far from optimal positions).

What happened blew me away.

The boat started to behave. It felt like the heel/drive -ratio (or whatever you call this) was way better than before, both with three and four panels. The change was also very apparent on the rudder pressure. The difference in the aggressive motion in following wind in a swell was also signifcant.

All in all, the boat sailed better and felt like a different boat.

What happened here?

The top panel has very little camber, maybe 2-4%. In this experimental sail the top panel has significantly more, and the shape is also more horizontal. 

Is this the main component? Or the reduced twist (with the top batten now sheeted instead of batten two)? 

Or is it just the sail area vs height, that reduces heeling?

The Galion has about 50% ballast, but a very modest draft (1-1.1m). Could it be that this boat just loves the drop in the sail height vs sail area so much, that the change is this dramatic?

I was going to leave the bottom panel away for the next season and cut about 70cm of the mast. Now it feels like I should drop the top panel, which would make it possible to cut even more (and further reduce the heeling momentum).

Aesthetics aside: what kind of  risks or potential problems would I get? More stress on the yard?

What would I lose (along with Unalome, my sail insignia)?

Woul it be possible to gain the better sailing qualities while keeping the top panel?

(Disclaimer: the effect might have been caused by my misintepration, my mistakes while sewing the sail or doing the sheeting arrangement, me being a lousy JR-sailor or some other things.)

Hypothetically in a situation where a fast release of sheets is needed. The amount and number of sheeting rope (1x5-part vs 2x3-part in my case) also makes it more possible for some kind of entanglement in a situation when this is not a good thing (like it ever would...).

But as I said, I think I gained more.

Tested a two-part sheeting arrangement today, with good results.

More hassle, in some situations maybe even dangerous, but I (and the boat) like this better. It’s nice to be able to control the twist (to a certain extent) and also to spill some wind on the upoer panels if needed. The two-part sheet also seems to act more kindly to the upper part of the sail and mast - because of thet sheeting angle, the top panels were easily pulled too much downwards and they even made the topmast bend a bit, if the skipper was acting carelessly. With the two-part sheet this is easier to control also.