I'm sure the guys at FI know all about your SX.  They may also be cheaper.  The only thing I would worry about is thier motors at least look physically different than the RE motors.  If that is the case, thier soft-parts geometry might not be optimimum for your SX motor.  I also highly doubt they would custom one-off you a soft-parts kit that was ideal for the SX.  I could be wrong, they could be better, but I wouldn't count on anything.  Also, it is doubtful you would raise power handling in a major way.  If it were me I would pony up and have RE do it, otherwise it would be more of a gamble....