Hey Gav,
My 4L60 is out of the Dragster at the moment getting the "fully sic" treatment.
I'm getting a trans-brake installed along with strengthened internals, more pump pressure, bigger servos etc. I'm also getting a super tough 3rd 4th clutch pack and billet servo, which will enable WOT 4th gear changes. The Dragster changes into 4th just after the 1000ft mark and the change is very slow and sloppy, he reckons it will shift just like the other gears....
Here is a short video of it running, you can hear the sloppy 3rd 4th shift right near the end of the track
http://www.youtube.com/watch?v=WAWubKvpuz4
I reckon a trans-brake is just the thing for you....