I've kind of follow the kit plane industry as I'm interested in one myself someday.
The Titan Mustang is really underpowered (when built per plans with a Rotax 912), to me it's like the folks who put a Ferrari body kit on a Pontiac Fiero. For a 2 seat tandem kit plane you'd be much better off with a RV-8. The RV has more performance in every respect, and it's safer (fixed gear, thousands built)
And if you want more performance then the RV-8 in a tandem, look at the F1 Rocket EVO.
The Thunder Mustang IS a high performance kit, can easily be built to outperform the original with either a V8 or V12 engine. Problem is those kits are no longer in production, and the prices for the existing kits are astronomical. Last time I looked (over a year ago) there where rumors they where selling the molds and tooling to someone who was going to start production again, but I don't think it ever happened.
Personally I want to build a Super Cub. Unfortunately all of the decent Super Cub kits cost 3 times what the "go fast aluminum" kits cost.
EDIT: Looking at Titan's website, the Suzuki or Honda V6 engine option is new. Last time I looked a Rotax 912 was the only factory option. While the V6s would certainly give it more acceptable performance, you then run into all the issues involved with running a liquid cooled auto engine in an aircraft (radiator, PSRU, intake heat, etc). I know auto conversions have been successfully flown a lot, but I personally would feel much more comfortable with an aviation designed engine. If someone where to built a 3/4 scale mustang kit utilizing proven Lycoming or TCM powerplants, then I'd get excited. But even with that, you still have the worry of retract gear.