140k is not too much RPM for this turbine.....The AMT NL Mercury in the HP configuration (High Performance) turns 149k----The AMT USA 180 SP (Special Performance) I believe turns around 151k.
Nice work on your engine. After you get accurate thrust readings, if they are indeed as much as you think for your given RPM, you may want to call AMT and see if they are interested in your mods.
Kevin