The Up-Pipe is deffinitly a "Since your in there might as well do it".. mod.. LOL. You have to pull off the Down pipe to replace the Up-Pipe so you might as well change it now since you want to go catless.. That cat will slow air flow a bit so might as well get it out of the way.. As for the pipe Dia, keep it the OEM size. You want to keep the Velocity of the air high so don't enlarge the pipe on the stock turbo or you will lose pressure. Well, you will lose air velocity..
As for the BOV check around, there a TON of posts about this so don't take my word for it. Anyway, the BOV will release pressurized air into the air making that whosh noise everyone wants. The propblem is that the stock ECU has already metered that air volume and added fuel to mix with it in the combustion chamber.. Thing is instead of plumbing it back into the intake system like the stocker your freeing it from your system. So what you say, what about all that extra fuel you now have.. A motor running rich doesn't make as much power.. So if you want to hear that sound more do an elbow delete and free up the intake flow.
02 WRX.. "It's fun when it runs.. Problem is that it hardly ever does..."