No, lots of EL84/6BQ5 guitar amps run those tubes above
300 volts without any problems, including Vox AC30. It's
really the heat, not the plate voltage, that can be hard on those
tubes when being run in class A.

Actually, you probably can run that tube slightly hotter, by
reducing the cathode resistor value a bit. It's idling at 36 ma
and the plate dissipating around 11 watts (I'm not considering
the screen current draw, to keep things simple).
EL84's are rated for 12 watts max. plate dissipation.
Try and increase the idle current draw to around 40 ma. 

It sounds like you are within the current and power dissipation
ratings, so it should be ok. I don't think 7 or 8 volts is
excessive. I've heard that AC30s run very close to class a at
315 volts.

I'm working on a project right now with parallel SE 6bq5 and
I'm running them at 340volts. It's likely though that I'll put
7189s in there as soon as I get some.