Just and FYI... there are many times that SQL code simply can't be tuned for performance and will need to be rewritten... sometimes, from the ground up.... 1 to 4 weeks of part-time nights and weekends may not produce what you want.

And, once you get it fixed, then what?  The same people that wrote the performance challenged code on your current staff are simply going to write more performance challenged code and you'll need to find a temporary Ninja, again.

It would be a far better thing to have your people get some training in writing good solid set based code to stop the cycle of performance problems you are setting yourself up for.