Lowering the TTVA value will cause it to shift earlier, raising the value will delay the shifts. The TTVA angle effects the boost pressure and boost valve, which effects line pressure. Drag the TTVA angle out for too long and you're causing the line pressure to not rise as you're increasing the load....not a good thing to do. You also have a few other factors that messing with the TTVA can effect and if they're not adjusted correctly you get some weird up/down shifts and also a bunch converter locking/unlocking.
Being that the 48 doesn't have great line pressure as it is, unless the trans is modified, I DO NOT recommend lowering the TTVA angles for the sake of stretching out shift points. Making the shift points occur at higher RPM's is no issue, as you're increasing TTVA angle and therefore line pressure, which is a good thing. But again, you have to take a few other things into account otherwise you'll still get some weird tranny behavior.
On mine, with the single 88, I need the shift points much higher in the RPM range to stay on top of it. This is what mine looks like (minus the actual values and scale). Just for reference, this shifts at 3900 RPM's at WOT. You also have to look at your 3-4 up and down shift points and your converter lock and unlock, otherwise it'll be all over the place. You have a bit of room to increase the max TTVA angle also, 50* is not it's max travel. If you set it too high, you'll know it, it'll toss a code for the TTVA being stuck. That's how you know you've reached the max angle for your particular trans.