NO! I'm not saying its dangerous.  Just that you are starting to lose performance and efficiency.  Why buy a 50 pound thrust motor if you are going to undersize the wire to where it only performs like a 40 pound thrust but still have the same drain on the battery as a 50 pound motor. 

One thing that should be clarified is "Just how often do you spend time running at 100% thrust".  A lot of people may rarely ever run at 100%.  I did a lot because on the lake I fished, once the water warmed up, I would spend a lot of time chasing schooling bass feeding on schools of shad and running from one spot to the next that may be 100 yards apart where there start breaking the water.  When casting and just running around at 10% to 15% power, if your TM power cable is one or two sizes too small, it would never make a difference.  However, if you run around in cove from one point to the next, and use max power a lot doing that, then it can make a big difference in how the motor performs.