I've never seen a streetable B series engine making 300whp on pump gas...
What aspiration type you choose should really be dictated by the kind of racing you want to do- drag racing favors tons of power and is very forgiving to boost lag, while road course racing favors a smooth power curve with no lag and lots of control in partial throttle situations. I'm a road course kind of guy, so I stick with all motor- especially in the front wheel drive platform.
Which do you want? Boost is a cheaper way to make power, but the power curve you get out of it might not be what you want.