My theory is that Fender very conservatively rated the newly designed 2x10s in the Rumble 500 combo at 350 continuous watts simply because that's what the 500 amp was rated at 8 ohms. (This was about the same time they decided very conservatively to put the same 300 watt continuous 15" speaker in the 115 cab and the Rumble 200 combo, where it can only ever actually see a maximum of 140 watts continuous!) The 210 cab came about a year later and also was given the very conservative 350 rating, probably for consistency.
Then, a few years later, with lotsa real world experience on these speakers, comes the Rumble Stage 800 combo needing speakers for 400 watts at 8 ohms. By then, they are confident that the same 2x10s can handle it just fine. So, should they re-rate the 210 cab and the speakers in the Rumble 500 combo?? Nah!! Too confusing. And when they needed a 400 watt extension cab for the Stage 800 what do they do? Take the same 210 cab and only change the cosmetics to match the Stage combo! It's the same speakers in the Stage combo and the black 210 cab, both rated 350 watts on paper, but actually taking up to 400 watts from the Stage 800 amp!! Not a problem. Still within warranty!
Worried about those extra watts between rating and actual use? I wouldn't be, not one bit. But, if it bothers you anyway, then just add another extension cab! The Stage can run just fine with TWO 8 ohm extension cabs, there's even an impedance switch for setting that. (Yes, the amp is running at 2.67 ohms that way. No problemo!) And the Rumble 800 HD can run at 2 ohms, meaning up to FOUR 8 ohm cabs!!!
Now the Bassman 800 is a little different. It runs at 500 watts at 8 ohms!! I wouldn't use anything less than a 500 watt Rumble or Bassman 410 for that!
Hope this helps.