Depends on how you drive it, as usual. I got a pretty constant 24 mpg on my daily commute (80/20 hwy/city with some traffic) in my 10 GT, but I did drive it a bit hard at times.
The few times I drove it on the highway at 65-75 I got up to 29 mpg. I know a few people on here (Carrie - Vibenvy and her mom) get pretty good mileage (upper 20's) from their 09 GTs, certainly better than I did. See the MPG thread on here for a lot of reference info.
If you were getting 43 mpg in your 03 GT, was that all downhill with the engine off???
I've only gotten about 29 hwy in my 03 GT (23-24 city) and my son just gets up to 35 hwy in his 06 base.
Good choice on the red 09 GT though. I wish I still had mine, and maybe one day I'll find another, but with a manual transmission too. I did not like the auto trans though coming from a 5.0 Mustang, but had to get it for my wife & daughter. It did handle pretty well though and I liked that aspect.
Good luck with the new one!