Another reason I didn't want to relocate to CA!
Check ebay, I'm sure you can find a decent exhaust for your 525. Also check craigslist to see if anyone is parting one out in your area. There is no way I would be paying full dollars for a stock exhaust.
Your aftermarket cats are probably the problem, are they supposed to be high flow? That could possibly explain the difference betw. 15 & 25 mph.