the Leadtek should be able to handle higher clock speeds too, since it has a larger than normal heatsink. the GT's have higher clock speeds, so after a softmod you should get those speeds.
if not, after you softmod it, you can underclock it, or softmod it back and it will sill have the 16 pipes.
one of my friends has an X800 Pro (12 pipes), he softmodded it to an XT (16 pipes) but it overheated with the higher clock speeds (his cooler wasn't that great though) although he could just softmod it back to the pro, while keeping the 16 pipes