Think of it this way, an engine is an air pump. Pulls oxygen into a combustion chamber and ignites it to make a charge. The by-product of that charge also has energy "exhaust gas". The more rpm the engine is turning, the more air is being pumped, the more energy is exiting the engine. Some turbine wheels just need more energy to spin them. Of course there is more to an efficent setup like cam, c/r, head flow, intake volume, etc... to get the right stall for your setup.
As for lock up I was told in another thread that it makes for a more efficent drag setup, I dont know about that though.
I've always been told lockup effects crusing. Lock up converter uses a clutch, do away with that clutch and it makes for a stronger unit. I would not put a non lock converter in my car unless it were drag only, but thats my prefrence.
Go to youtube and find out how tq converters work if you dont know already. A lot of great videos about tq converters.
btw...what turbo are you looking at?
and btw....I'm running a 3500 stall and it does good on the street. The only difference while crusing is you have to give it a little more throttle to get it going. The difference at the track is about half a second in the 1/8th mile.