you beat me to it
transgo does make a kit where 1,2,d (3) can all be held for whatever rpm but once you're in (d), it acts like a normal auto
i thought of a full manual valvebody for my rs (just to liven it up a little) but somebody suggested this and it seems the most logical
i think the only difference between them (aside from what i already mentioned) is that the semi-manual (we'll call it this for easier explaining, not sure of the real name of the kit) may not be able to shift into 4th at wot
i assume the full manual can since (d) acts as only 4th gear, but the 700r4's won't shift into 4th at wot from the factory
this could be remedied though by buying a 15$ part from gm which does allow wot shifting into 4th

so, imo, go with the semi-manual
it offers both types so you can pretty much decide "do i want to go manual or auto today?"

 

Reviving an old thread here guy's but I was found a link on a site for a homemade Trans brake in the 4l60e using the shift soleniods, but the link doesn't work and the the site owner could find the info anymore, any one know about this??

p.s it could have been Brents site.