I installed a B&M shifter in my 2005 GTO. It was about $180 and was quite an improvement over the stock unit. I also looked at the GMM and thought it was a lot of money for a shifter. I would highly recommend the B&M unit and I think you'd be happy with it. The throws are shorter and more positive now and the installation wasn't too much of a job.
I've got a B&M and didn't like it until I had my transmission overhauled with bronze and carbon parts. Now it's shifting well, with no grinding in 2nd (synchro) and being able to shift into reverse with no problem. So far so good but only have about 300 miles on the transmission so time will tell.
1-7/8" ARH LTs, 3" Magnaflow exh.,Spec twin disc clutch, 402 stroker , Fast 102 w/Nick Williams TB, 1000 hp shaft & axles, cam, fuel & suspension upgrades. 500 rwhp, 484 rwtq