I replaced my previous Gill with a Concorde sealed Absorbed Glass Mat (AGM) type battery about 3 years ago. Battery box is clean and starting is a breeze. I did find out a slight issue with the change over. That is the battery box needed a slight mod to get the cable spacing correct for the Concorde. I had to make a patch over one of the holes that needed to be moved due to the battery configuration of the Concorde as compared to a Gill. I then re-drilled the hole, made and riveted two new phenolic resin insulators to the box for both the positive and negative cables. All mods to the battery box were inspected and approved by our IA.
As part of the change, I reworked the ground (negative) cable termination to the airframe that was somewhat flaky with resistance that was on the high side due to the bolting arrangement, dirt and corrosion. This cable termination is just adjacent to the battery box. Another area to make sure is properly connected are the grounding straps from the engine to the engine mount and airframe. With the long run of cables in a Comanche a poor ground connection at any point can negate any gains from a new battery.
Now, the Sky-Tec starter turns the engine over like there is no compression and starts occur in about 2-3 blades. Can't say enough positive about the Concord battery and making sure you have a great ground connection to the airframe.