I spent a fortune on my old ST170, hands down the best modification I ever did was fitting a lightened fly and stage 3 clutch and clutch plate, next was retarding the timing -6 and then a short throw shifter to speed up the gear changes.
I had underdrive pullies on mine and they definitely helped and I never had a problem with harmonics.
With all my mods on the car I could just about get my 0-60 time into the 6's and pulled a GPS top speed of 147mph on a private track