The quick answer is probably 10bhp more than you have now
There's nothing wrong with 10bhp either - as long as it's associated with very little weight and you're willing to put up with the bikes limitations in one area to gain advantages in another. I can't imagine I'd get much joy out of a 1200GS with only 10bhp but my 88kg Suzuki seems to perform well enough on it.
One thing I have noticed over the (many) decades that I've been riding long(ish) distances is that the power required to maintain "equilibrium" with the other vehicles on the road has been going up. When I first started about 30bhp would have you amongst the fastest vehicles on the road. Not in the fast lane of the autobahn maybe but just about everywhere else. 60bhp (a 750 Honda for example) would have you well in front of just about everything. Now my 60bhp bike feels about the same as the 30bhp one did. You need that power to be able to keep up with the traffic and 120bhp (a GS (roughly)) to stay in front. I'm not talking about speed limits or things like that but the way cars handle, accelerate and respond - up hills for example.
So yes, if you want to go out with the big boys you will need 120+bhp and 1200cc and I've seen GSs and Ferarris playing nicely together on alpine passes near me. If you are happy to do your own thing, ignore the traffic and go at your own pace then 60bhp or (much) less is fine.