I can't help with the shim in the sear idea as I have never done it on a springfield. Though it sounds interesting. That may help the creep.

Heavey pull on a musket lock can often be helped in two places. The main spring and the sear spring. I have seen some pretty stout sear springs. If it is an original spring as issue as you suggest, ans not hardly ever used, it may have never been "broken in". 

I know you don't want to alter the original, but repro springs that can be honed or thinned down often help trigger pull on these things. They are simple to replace and the originals can always be put back in. 

Merwin