I don't think a huge change in game performance will be noticeable. Most changes don't directly concern fortress mode, but, depending on when history is simulated, the changing of seasons/years will probably be slower. That said, this update also brings rewrites to old parts of the code that weren't fit for his purposes anymore. (Which probably translates into improvements, assuming that Toady's coding skill goes up with time)
Don't worry, skills can go over Legendary Codedwarf+1 even higher with time. Toady's probably between +5 and +10 by now, one can only wonder how big his skill gain be in the next ten or so years. Sadly, he's the only one working on it, so there's no one else to have the code menacing with spikes of
bugfixes and adorned with
optimizations, which means a lot of his time is also invested on raising the codefixerdwarf and optimidwarf (Optimus Dwarf? Urist Prime?) skillsets, making the entire process slower.