There was an article in Pilot years ago by someone who took a Turbulent to 20,000' just to see if it could be done. I'd like to re-read that if possible.
Highest in a helicopter for me was in a AS355 at 14,000' in formation with another, the handling and performance were not the best, normally we cruise the jet at FL410-FL420 but that can be quite weight dependant and an auto-pilot walking out could make handling a touch awkward..We practise that in the sim and once a month have a go at doing it manually, the workload is surprisingly high.
I managed 20 500 over Aboyne in a Standard Cirrus glider, took hours for my fingers and toes to thaw out, but the view was stunning.
My favourite was in a Cessna 182 into Leadville Colorado 9927' AMSL, you get a certificate and I've still got the t shirt.
SND