In mod no one cares about the motor rules to be honest so your 100% right. Even at the ROAR Regionals this weekend my buddy ran 2wd mod and 4wd mod with the Castle systems and no one said a word about it.
To be honest what track is going to chase away a paying cutomer that is paying for 2 entries. Tracks need money to stay in business and pretty sure no track will send paying customers away.
The 5700 is nice for the days you can lay down some extra power.
I'm getting my 4wd car ready to go in the next week or two.
It will have a 6900 in it for right now til I get my 5700 back from my buddy or til I order a new 5700 motor.