I reflashed RC3, enabled both software/hardware offloading and... it looks like you're right! Or at least it seems to have helped somewhat. Quick before and after comparison:
I consistently get scores in the 500s now, up from the 300s. Obviously this is still not the full 900-950 I would expect, but to be fair it's a weekend evening so it's entirely possible that my ISP is just overloaded. I'll do some proper testing once I'm able to, and study what the offloading options do (and why they're disabled by default) in the meantime.
Oh, since you're here, I figured it's worth a shot to ask - are there any "magic" options I should know about that make, specifically, long-range 2.4G wifi speeds usable, besides the obvious? (free channel, 20MHz width, high transmit power). If this is a hardware limitation of the device itself then I'm willing to accept that, but I find it ridiculous that I get nearly an order of magnitude longer reach with a 10 year old 4/32 router that has 2 fewer antennas.