To put the Twin Comanches' single engine performance into perspective, take a look at the following figures I got off the net. I acknowledge that some of the figures may vary slightly from some manuals but never the less will be close enough to give a good comparision.
PA30 260 fpm
PA31-300 Navajo 270 fpm
PA34 Seneca II 235 fpm
PA44 Seminole 180 fpm
BE76 Duchess 235 fpm
BE58 Baron 390 fpm
Cessna 402 225 fpm
DA 42 Twinstar 160 fpm
While none, except perhaps the Baron, have startling single engine performance I think you will agree the Twin Comanche compares pretty well.
The PA30 (39 if you can score one!)
Well actually the PA30 outperfoms the PA39. Long story short, counter rotating engines are not as efiicient as non counter rotating engines. It's to do with the slip stream from the props and the flow over/around the airframe. Apparently Piper increased the power on the C/R Navajo from 300 to 315 to get the same performance as the non C/R Navajo.
The only advantage that the PA39 has in my eyes is the more modern panel layout and you can be that in the PA30C anyway.
Apparently the PA30B was be best performer however my choice would be the PA30C to get the modern layout, but even then there are STC's to change the old layout to the new layout.