I always go to Grand Auto in Thornton on 104th west of I-25 (I know the svc mgr, the advisors are OK.) There is also Johnson Auto plaza in Brighton -- We buy a lot of parts from them, but have not used their service department.
I don't know the number for Johnson, but Grand is 3/460 8000, just ask for GM service. but beware, the advisors are only OK in my opinion. Hope that helps.
'09 SBM GXP A6...CAI, Headers & Exhaust, mild cam. - 13.04 at 106.34MPH at Bandimere,
'08 IOM G8 GT...CAI, Cam, Headers, Tune, NOS -11.80 @ 122.2 MPH in Pueblo.
'11 Cadillac CTS-V Coupe
'99 Grand Prix GTP...some mods...Best ET was a 12.80 at Bandimere,
'94 T/A VorTek Super Charged & NOS - Best ET 10.2, quickest MPH 134.9 & still street legal!