Fwiw, no issues so far. Out of fun I created a medium sized project (a bunch of sampler instances, EXS and Kontakt mixed) with no regions on any tracks (hence template style) in 4.4 and closed Logic. Starting 4.4 with that project takes 45 seconds, with 4.5 it's 8 seconds.
Obviously, once you actually use all those tracks, the returns are diminishing, but it's a pretty damn impressive thing in case you're using large templates (or rather avoided doing so until now).
Might be a good idea to either hide large portions of such a template or put things into folders/stacks, because once you click onto a track accidentally the used instrument will obviously be loaded.
I may not exactly profit from this feature too much myself - but I can clearly see how people into things such as creating larger orchestral scores will just love it. You could really just load everything into your template without any waiting time.
Gotta say that this is really excellent. Not sure whether other sequencers are offering a similar thing, either.
2010 MP 2x2.66GHz, 16GB, 500GB SSD System/Logic drive, Zoom UAC-2, LPX 4.2, OSX 13.6.