They did run the Ginetta LMP3 as Watkins Glen and it was quite slow - but it's probably one of the worst LMP3s. Incidentally
they also came out with this. Which although not mentioned as a LMPC proposal, with an LS3 engine seems like it was certainly something to think about. I hope they get the chance to evaluate a better performing LMP3, like the Ligier.
There has been talk about having a spec LMP2 car as a replacement for LMPC.
Mentioned here.
But yes,
they did put in development upgrades lightening the engine by 25 to 30 pounds, ORECA making aero balance improvements to the chassis, and the electronics package and system being modified include traction control and other elements - to be in place by Sebring.
Money to help run the series is not a bad thing. It's an investment to help it be run well. Balancing BoP has a real tangible cost. And besides, it's not like the GTD class is hurting for entries or diversity: Audi, Dodge, Porsche, Lexus (soon), Aston Martin, Ferrari. Mercedes might not be on that list now - but when they are ready to be, they will be.