Konrad, I wouldn't bother folding the zipper ribbon and sewing as you suggest. Frankly, if I tried the bag on, loved it, loved everything about it, and had to have it... I would just cut the zipper out and start fresh.
There's almost no comparison to the SummerLite. Maybe I'm getting older or grumpier, but I'm finding that my lower legs need a little room to move in the night or I get stiff... not a problem with the SummerLite. I prefer the circumferential baffling on the SL, because I regularly shift the down to an even 50/50 distribution and use it as a quilt. I feel like there is more than the spec'd extra inch of chest width in the SummerLite; I can easily layer up in the SL, but didn't feel like I had that kind of room in the Speed.
The footbox of the Speed 32 is sculpted more nicely, but I'm not sure I noticed any functional gains. I liked the depth of the Speed hood better, but prefer the drawcord arrangement of the SL. I don't feel like there's a ton of difference in warmth between the two, but if someone put data in front of me showing that the Speed is a few degrees warmer, I could believe that. I actually prefer the shell fabric of the Speed. Would love to see WM do an uberlight bag, maybe sewn-through baffles on 5" spacing, full-length #3 zip, and a ~0.65oz shell.