Depends on the corvette... Ultimately, Corvettes are capable of faster lap times there IMO. It's a horsepower track, and even the most powerful track miatas don't have that much power compared to the world of Corvettes.
The fastest miatas on the West Coast have gotten to ~1:27 flat.
Miata time records for Willow Springs are listed here:
Miata Track Records
TrackHQ lap records shows two Z06 T1s on Hoosier A6s doing a 1:26.8 and a 1:27 flat. These are race cars, but they are limited in power for their class - a Z06 built for Corvette Challenge in the Super Modified or Unlimited category could easily have much more power, but then again the prep level/suspension for the T1 Vettes is probably higher caliber than what you'll see in the Corvette Challenge...
Link to THQ list: Top 25 Willow Springs Lap Times
Short answer though, if one of those T1 Vettes decided to not run T1 rules and went and switched to a 550hp motor (which is easy for an LS1 to make), it would run circles around any of the fastest miatas.
I was there last weekend and did a 1:35.2 in the summer heat. I think it was good for a low 1:33 at least in winter temps. That is with 170whp and Street Tires though.
Another angle to look at it from; you could compare the fastest pro-level cars of each make... For miatas that's Playboy Cup MX-5s. For Corvettes that's probably ALMS. I'm sure the ALMS Vette would beat a Playboy MX-5 by 10 seconds easy.