I don't see any reason for him to get an LS tranny if he's SCing. The reason that you would want an LS tranny while turboing is this:

Boost kicks in at about 2500 - 2700 RPMs. With a GSR tranny, if you're on it and shift, the tranny is geared to keep you inside of vtec so you keep your power up thus keeping you above 4200 RPMs. If you have a turbo, you'r losing that range of 1500 RPMs worth of boost. On the other hand, the LS tranny is not geared that way because it doesn't have vtec to keep you in. Because it's not worried about vtec, the gearing drops you below 4200 RPMs and will allow you to utalize move of the boost power.

With a SC, it really doesn't matter imo cause boost is 'always on' and you can't waste boost.

That's my take.