They do in adventure mode at least, I haven't gone back into campaign yet after just beating Maltheal last night. I feel like they would/should, are you sure?
Yes, the other modes are scaled, but acts 1-4 didn't seem to be in storyline. I was running T2 with my brother's new characters PL'ing him (which works very well FYI) and getting level 66 drops (being 70) and everything was dying extremely fast. I used to run T2 at level 60 and go through it easily, but not have trash die just by being in my SW. I didn't get to try T6 yet again to see if it's feasible.
Edit: Tried storyline mode again with the earlier acts and they weren't as easy. I'm guessing this was due to the lower level of my brother's character and the game scaling the difference.
I'm sure the experience base amounts in these acts are much lower than adventure mode and A5, but the bonus may more than make up the difference. I'll test it today when I get home; break out the spreadsheets. ^^
After I bit the bullet and dropped my WKL (RIP!) and switched my build up a bit I ended up doing far better in A5. Completed it on T1, Maltheal did take a few tries. That's one hell of an annoying fight for a melee because of those clouds on the ground.
Yeah, I'm going to have a very hard time giving up my gear. As it stands now, I'm very built on having 50%+ CC and 2.8 APS+. The small bumps in necessary stats from PL aren't enough to compensate for the loss incurred from moving to basic higher level gear. I did replace my Fist of Azturasq for a rare 1800 DPS sword with 500+ dex/vit and socket and that bumped my damage by 200k, but it took a while to find that one. :/
I enjoy the Malthael fight as a melee. I've become complacent for standing in just about anything that bosses throw at me, so the need to actually move in a fight is refreshing. Though this is likely just due to being undergeared at 70 and that'll probably change, it's still a welcomed challenge.