The figures for the SR20 sound really low. The one I flew (admittedly a little) was indicating 150 kts at 75%, but I'm not arguing your numbers, just surprised. I don't go by sales brochures, I make them for a living - I know they lie, just like I do...
The Commander I used to fly would do 145 kts @ 6000 feet and 12 GPH which is higher than book fuel flow, and slightly less efficient than the TB as I'd expect.
I'm surprised the SR isn' better in fact. However, what you're saying is that speeds and fuel flow match but that's with gear down on the SR and reracted in the TB I presume. How does the TB perform with it's legs down? The Commander basically stops in its tracks when they're lowered!