Jumping into the discussion late;
I recently purchased a Mech Tech carbine in 460 Roland, but have not test fired it in 460 yet. (I know, not exactly what you are asking.) But I plan to do a side by side comparison with the 45 APC, and I will report on the difference through the 16" barrel. Ultimately, I think the Mech Tech will find a permanent home on top of my P-14.
Also, I was flipping through my Brownells' catalog a month ago, and recall seeing a number of drop in 460 configurations listed. Check their on-line site to see what is available.
http://brownells.com/
Marty