If it's a clay-based track, did you try the clay oriented-compounds? Since you mentioned soft and super-soft compounds, you should be aware that those often have the issue of folding up or deforming when used on actual clay.
I would first try these tires up front:
http://www.amainhobbies.com/product_...-Buggy-Tires-2
IMO, buggy setup is at least 80% tires. You should probably match those with a similar set of rears in MC at the same time.
Then, failing that, I would look at actual setup. On-power steering can be achieved two ways. I would, for 17.5/blinky on a very large track look at getting more of it by loosening up the rear first. For example, reducing rear toe-in would both add steering and make the car a touch faster on the long straights as a bonus followed by just general stiffening of the rear oil/springs. However, if you don't have any rear bite to spare, I'd start stuffing weight in the front end. But, definitely start with the tires.