Congrats on a good V8 GT coupe!
The water pump, thermostat, even cracked coolant bottles and dodgy coolant bottle caps can give leaks and not a major job, fingers crossed it's something like this!
Unfortunately the V8 will only give a bit of extra power as it's a naturally aspirated engine. Big power increases come with forced intake like supercharger and turbos. You may get an extra 20bhp with exhaust, intake and a map may give a few extra bhp ontop. These engines do like a supercharger and kits are available for around $25k!
Also you may have lost power due to carbon in the intake which these engines being direct injection do suffer from. It has been known to loose about 75bhp due to carbon build up.
So get the coolant issue sorted, then have it checked for carbon build up and go from there.
I have the V6 3.0 supercharged S5 with around 460bhp, but I don't have that lovely V8 sound!