So far in testing with the LM-1 spec kit, 21.5 2S on medium to high grip carpet works well. 17.5 2S is a handful on the same type of track and detracts from good racing. On a long high grip track it would be a decision to run 17.5 2S. 13.5 2s would work on large outdoor venues, something we don't have locally. 13.5 1s WGT is still a rocket in comparison to the 21.5 2S in the same conditions, I'm guessing it's the 250+ gram difference.
*** If SP wanted to corner the market, they could put out the FRP lower chassis plate to accept other front ends: CRC, Associated, Tamiya F103GT, etc. At $20.00 for a replacement, they would essentially own the class.***