The P-51 Mustang cost $50,000 in 1945 dollars, which if you believe Government statistics, is about $650,000 today. According to the specs of each, the top speed of a P-51 is about 465 mph, while this "Super Tacano" is 368 mph.
Why not just build more P-51's and upgrade the avionics????
The Merlin (and all piston engines) are more maintenance intensive
The P&W Turboshaft engine has a 800hour Insepction and 5000 hour TBO (time between overhaul)
Where as the V1650 (Packard built Merlin) has a TBO of 320 hours
There was an attempt to build a Turbo Prob Mustang at one time
http://en.wikipedia.org/wiki/Piper_PA-48_Enforcer
I suspect most pilots would far prefer the turbine engine rather than the old piston V-12. There really is no comparison in reliability.