The boatload of torque the 3.9L has is available at a very low 2300-2500 rpm from what I've read.
I seriously doubt that the DOHC's torque peak is below 3000 rpm (DOHC's are inherently revvier) and the difference in quickness will probably be negligible between the '06 and '07 GTPs.
Hmm, just joined here from the GA side 'cause I was bored, LOL.
About the LY7 (3.6L DOHC)... As much as the pushrod ppl (I'm one of them myself, lol) might like the LZ8/LZ9 (3.9L "3900" OHV), the LY7 is quite a nice engine. I'm really glad/excited GM is offering the LY7 in the G6 and Aura for 2007.
The LY7 reaches it's torque peak at just 3200rpm (251lb*ft). It does kinda spike downwards slightly to 4000rpm but it slowly goes up again as it goes towards 5000rpm. The torque peak is not below 3000rpm but it's actually at a lower
RPM than the 3900's torque peak.
The LZ8/LZ9 on the other hand reaches it's torque peak at a [relatively] lofty 4600rpm (240lb*ft). That's 11 less lb*ft and 1400rpm later.
Now you might think the 3900 has more low-end torque in general though, right? Not so.
The LY7 has very similar torque levels as the LZ9. In fact, right from the lowest RPM where torque figures appear on the graphs (looks around 1200 RPM to me), the LY7 is already making just over
200lb*ft of torque. The LZ9 on the other hand is at perhaps only 180lb*ft at that point.
At 1500rpm, the LY7 is making perhaps 210lb*ft while the LZ9 is just breaking into the 200lb*ft level. At 2000rpm they are both making what looks like a similar amount of torque (~220lb*ft). The LY7 however continues more rapidly to it's torque peak (gets there at a lower RPM as discussed above) and
it has more torque overall.
The LY7 does take longer to get to it's power peak (252hp @ 6300rpm v. 240hp @ 6000rpm) but looks like it has slightly more power at 6000rpm than the LX9 does anyway.
The only thing the LZ9 appears to enjoy over the LY7 is that the torque curve looks smoother. Oh, and of course, the fact that the LZ9 can be mated to a manual transmission whereas the LY7 is automatic only.
Anyway the proof, if you need it is in these graphs below, both SAE J1349 Certified
3.6L DOHC LY7
3.9L "3900" LZ9
[Note there is a difference in the torque scale on the side, so be careful when comparing them, you have to take certain RPM points and compare the torque levels, like I did above.]
I'd agree with you that performance of the cars will be quite similar and probably not worth "worrying" about if you have an 06; but make no mistake the LY7 is no slouch. Compared to the 3900 with 300cc more displacement, it's breaking the no-replacement-for-displacement rule if you ask me