b18b1 pieced together kit, going to run a 57 or 60 trim t3 turbo at about 10 psi. would i be ok running my stock exhaust with hollow cat for a little while? or should i get a 3" exhaust right away?

 

3 inch?? i would say thats a bit overkill for 10 psi boost. the stock exhaust replacement will help your power gain but 3" is not needed. go with 2 or 2.25" exhaust.

 

3 inch good for more than 300-350 hp

 

Guys while the figures you talk about relate to max power you forget that the best exhaust on a turbo is no exhaust,recently upgraded a mates Subaru RX turbo an engine far less efficient at spinning a turbo than a B18b1 and going from a 2.25" up to a 3" brought the turbo on boost about 1000 Rpm earlier and much more lively throttle response.
So big exhaust systems do not just add benefit up top if properly made,the Subaru one was handmade and has noticeably improved the car,one of the least likely candidates for such a system.

 

this guy is only going to be running 10psi. 3" is a waste of money.

 

3" exhaust is most definitely overkill for 10 psi. Atmospheric pressure inside that large piping will cause a restriction in itself due to the lack of spent gases. 2.5" at most. Unless this is a track only car which will see a high flow all the time and never need to worry about this.