I'm goin to replace my oem struts with koni str and adding my progress technology lowering spring that are on my 05 gt. should I replace my upper strut mounts with new oem or the stiffer gt500 mount?

 

It is worth the few extra $ to get the GT500 mounts.

 

I did the GT500 mounts when I did mine about 4 years ago. Well worth the few extra bucks

 

Go with the GT500. Not much more money than stock and better quality.

 

I can't speak to the GT500 mounts, but the stock units are infamous for making a popping noise with lowering springs. So I'd rule out stock. I went with Steeda HD strut mounts which have given me no problems and allow more range of camber adjustment but they are $$$