The tradeoff skews very heavily in favor of compression these days, at least if you're using a traditional HD.
Hard drive seek times are measured in multiple milliseconds. 1ms is 1 million cycles on a 1GHz CPU. You can do a lot in that time. For every seek you eliminate, you buy back millions and millions of CPU cycles.
As for your GIMP working set? The main gain you'll get there is from the other data in your system compressing, making room for GIMP. And besides, GIMP does its own paging via its tile cache, doesn't it?