It's not worth surfing at that level, in my opinion. I could run the whole park with minimal scraping, in a 60-gal River-running playboat, but I could not surf.
The most you'll get out of it, is better combat practice than a pool. I basically got my combat roll at Golden with low water. If you're bracing on the pour-over though, you'll definitely hit rock with your paddle. I would recommend messing around in Waterton over that anyway. If you lap Waterton, it's not really as long of a hike for each lap. Something like the first half a mile is flatwater.
I'm not an expert surfer, but I know I can hold a surf on other waves much better. Like the Glenwood playwave at 2200. If Shoshone is running, chances are, a decent river-right play wave in Glenwood is also running (which you can check separately). In my limited experience, that was the best surfing I've had this season, and I drove through snow to get there, and the second half of my wave fun was in the rain. There's nothing quite like it.
I would recommend hitting up the runs that are still running. Waterton and Shoshone, mainly, with the occasional random blessing like Big Thompson. Below the dam on the Big Thompson is a lot of Class 3, to 3+, at this level (400). It will turn into class IV though, so if you are wanting to avoid that, you'll have to know when to pull out.
My first season has been pretty productive, and I think I've gotten to the point where I don't like bothering with playparks unless their at medium levels or better. I'd rather do a nice III-IV run than mess around with low-water waves. Don't get me wrong, I'll make the most of my run if we're not trying to gun down it.
Just my .02. Take it with a grain of salt. My brace and roll is bomber, but paddling and all-around river-skill is lacking.