I think the TT +1 get's the room for 1 more by eliminating the floor plate. Putting a +10 spring would just take up the room of the +1. I'm not sure you would be able to get an extra round in there.
I have experimented with my TT +2 for 10 months now. No problems. If they work for 12+ months I will switch to the 43 for my onegun. I will then just change the springs once a year.