re: a 15t jump. For years I rode a bike with 50/40/24 with downtube shifters, and the 16t jump was doable, its just that its a bit too much for everyday riding. Im sure with friction shifting it shouldnt be a problem, it wasnt with the old lx fd on that bike. The time I changed my 50/39/30 to 26 posed no problem for my tiagra fd, but then this is 105 so who knows.
Personally, I think its worth doing the 36 mid ring thing, as riding loaded I figure 36 is a minimum for a 700 wheeled bike for having a more workable range of gear inches in the mid ring, where we are most of the time.
I expect the fd on the bike can be moved down easily on the frame, so if you go with a smaller crankset, it should be a fairly easy adjustment of the fd down a bit for the smaller rings.