so it depends a bit...
What transmission are you going with?
simply putting a brake in the trans with a tight converter doesn't mean you get into boost, you'll still be up against the stall speed of the converter wherever that happens to be. If you're going with a GM transmission, something else to keep in mind is that the stall speeds are based (usually) on what the converter will do behind some kind of small block chevy, and you telling them your car makes 350hp (for instance), you want a converter that will stall at say 3500... means they're likely going to build something that stalls around 3500 making chevy 350 torque along with 350hp, resulting in a lower stall speed, at least out of boost.
think of the transbrake as a means to hold the car still while you're trying to build however much boost at whatever rpm you're wanting to leave at (and with good tires, those numbers are likely going to be higher than you think). Have you had the car on a dyno yet to see where the powerband is?
tl;dr:
you can go bigger on the converter than you think unless the company has a track record of getting things right for small displacement turbo engines, and sure get the brake, why not. Usually you have to get a different style shifter for that, so keep that in mind as well.
