Well I plan on pushing it to 700whp and beyond lol. I would feel comfortable pushing it to 1000hp, however I don't see a point in making that much money unless your a huge name drag racer. 84mm isn't neccesary and to be honest if I could do it again I would've went with 81mm sleeves. I have said this before on here, but the main reason is should something happen and the motor needs rebuilt you have given yourself room for error. You have 81.5 all the way up to 84.5 (I wouldn't boost a B past 84.5mm personally) for rebuilds, where as if you start at 84mm your left with 84.5 (some people will tell you you can go to 85 but you have issues with headgaskets sealing).
I would have no one besides Mike Lasky build a bottom end for me. He is one of the coolest dudes I have dealt with. I chit chat with him on facebook randomly and he's an overall cool dude that knows his shit very well.
If you're scared of something breaking, then cars aren't for you. No matter how well built something is, it will eventually break, its mechanical and shit goes wrong over time. You just gotta use quality parts and hope for the best. If you need any more help just shoot me a pm i'm generally quick to reply