I've never operated in Europe but +20C at 9000'?
In PNG at 10,000'the highest temp I can remember was around +15C.
I think you should go with somebody experienced in high alt.ops and they should allow you to get a good feel of the A/C and the performance differences you will experience ie more TR and MR pitch higher turbine temps faster approach speeds sluggish response etc
Don't approach a ridgeline to cross straight on in marginal VFR if you can't clearly see whats on the other side but rather parallel or on a shallow angle close to where you want to cross.You should as previously mentioned always have an escape route that allows maximum benefit for minimum input.
Slow shallow approaches power in early minimum cylic and coll.corrections don't hesitate to abort and GA if in doubt.
The highest landings for me was Mt.Wilhelm PNG an RCH under 15,000'in a BA squirrel and H 500ER.
I pass on what was passed on to me
Regards