Hi Steve,
Many use HEQ5s on IIS with fantastic results.
Depending on the accuracy of your polar alignment, with the Synta mounts (EQ5 through to EQ6) you might reasonably expect up to 1 minute unguided exposure time. Above that time and trailing etc may be evident.
Good news is that these mounts respond extremely well to Autoguider commands so that your exposure times then become limited by the amount of light pollution at your site!!!
I regularly do 10min exposures with my autoguided EQ5!!
I would therefore recommend you to try autoguiding if you want to push through the 1 min barrier.
My setup is fairly basic - an 80mm achromatic scope, Phillips Toucam webcam, a Shoestring GPUSB adapter and free PHD guiding software.
Your setup beimg only limited by budget as there are many options.
All the best, good luck.
Check out my gallery link below to see my autoguided stuff!
Cheers
Doug