It's a good single valve linear using circuitry that used to be quite common back in the day.
(We used to take PL509 valves from old Colour TV's to replace them or build our own!)
As with anything shrouded with boastful information I'd say what you're getting is about right if you're reading through a typical watt-meter and antenna. With a sufficient dummy load and peak reading meter you'd probably see much more.
80-100 Watts maximum is about right for these valves with a sedentary life but again people used to push a lot more power into them to get 150W plus or so and slowly destroy the valve. (If pushing them hard a 30 second 50% duty cycle would be apt and usable.)
You could change over the valve if you feel you needed to but you won't be stripping them for free from Colour TV's nowadays. (Valves are flipping expensive today too!) I doubt however you'd see much if any of a difference. There's a good 800-1000 Volt or so on that anode so be VERY
careful if replacing it! (Unplug everything!) It should also be mentioned that you can get one hell of a RF burn from such units in operation too.
When used in Radio Amateur HF gear those valves produced their maximum power down at the lower frequencies slowly dropping off as you headed towards 10m. Bigger amplifiers tended to run the valves in parallel for more punch such as the RMS-HT200 and similar linear amps.
That's about as good as I can give you and I'd be happy with them as they'd run all day long like that.
There may however be suggestion of 'special' replacement valves, tweaks, mods or claims of much higher powers attainable and I'll leave that up to you to decide whether to heed such advice. (Myths especially radio ones die hard.
At least there's one thing.....it won't suddenly keel over releasing a cloud of smoke should your SWR jump up as you melt the antenna or 'accidently' put more Watts into it!
Plus you get that lovely 'warm' smell that only a valve can produce.
All the best,