Speed 2 is very slow for an imperial star destroyer. Didn't Han brag about being able to outrun these like it was kind of a big deal? If so, shouldn't they be at least as fast as a stock YT-1300?

That depends on how much credence you put into his bragging... I will note though that that's sublight speed -- he may well have been bragging about its hyperspace performance?

Two things.

1) Han never brags he can outrun a Star De stroyer, he brags "I've outrun Imperial starships, not the local

bulk-cruisers, mind you. I'm talking about the big Corellian ships
now. "

2) The Falcon is fast in Hyperspace, not necessarily real space (though it's pretty quick there as well.) He can outrun Star Destroyers in real space. but only just.

So yes, Star Destroyers should probably be at least as fast as the stock YT-1300. However the best comparison is it should be as fast as the CR90 Corellian Corvette. After all a Star Destroyer keeps pace with one nicely in the very first shot of anything Star Wars.

The Victory is way too slow and it also needs special rules allowing it to enter atmosphere (by the book, nothing greater than Sil 5 is intended to enter atmosphere).

The Victory is way too slow and it also needs special rules allowing it to enter atmosphere (by the book, nothing greater than Sil 5 is intended to enter atmosphere).

Even the SSD operated in atmospheres (at least in the EU). So, not sure why there is any limitation other than fluff.