How am I hitting a Taylormade "M" flex long and straight!?

Weirdest thing ever happened this afternoon. I am at the driving range with my grand dad working on hitting my driver straight for a tournament I have coming up on Tuesday. I just sold my original 10.5 R11 TP (aldila rip green eyes stiff) a few weeks ago to buy a Nike VRS with the same shaft as my old R11 (BIG MISTAKE). So I can't seem to get this new driver to go straight consistenly as my old R11, most shots are slices or hooks. So after spraying about a 100 golf balls all over the place I say hey grand dad lemme see whats up with that Burner ya got there? I pick up his 08 Burner with Taylormade RE*AX "M" flex shaft and I'm killin it straight down the middle every time! WTH? It feels so good just like my old R11! As for distance, the range ball were flying about 220-230 yards which at this point I'm happy with after having such bad luck with the New Nike club.

Now, my question is what would happen if I hit this same club with a stiffer shaft? More distance? Lower Trajectory? Both? I'm so happy to finally have the ball in the fairway again but my pride is hurting with a senior flex driver in my bag!!!!!

Little info about me

I'm 24 years old

6'5 300 pounds

I do allot of heavy lifting

10 index

SS 100-105

So basically I'm not exactly a senior flex candidate, right?