The race rule limits the width of tyres.(F26mm/R30mm) It also limits you to use touring body and no extra down force equipment. That limits the 1/10 touring car's speed just like F1. They don't want to see the R/C race become to a money game. If you don't join race,you can run wider rear tyres;outlaw pipe and VDS body. That is the main reason which limits the 1/10 speed.
Although the .15 do have more torque but the lap time are almost the same with .12 engine. Maybe they will allow you to use .15 in the future. But I think they will allow you to use .12 5port first.