can't be answered
too many factors, variances and differences between skiers.
no such thing as identical, especially when you put 'identical' skiers on significantly different skis.
a downhill ski is a balance of compliance and stability, as you go shorter the 'compliance' factor has to be compromised greatly in order to assure whatever stability that can be attained. At a real world 60-70 mph, the ski has to be compliant over terrain and yet allow strong directional stability. A much shorter ski will be greatly stiffer to assure some level of directional control and therefore greatly sacrifice compliance. Don;t make the mistake of assuming a cell ph app saying 60 mph is actually that.
I would terrified to take a 170 up to downhill speeds, even on a hard, well groomed stretch. A true downhill ski feels 'comfortable' at those speeds and gives much confidence to the skier.
On a true downhill course, I can't imagine a skier on any 170 actually completing the course without bailing out.
so taking the Schladming WC DH course - 3000 ft vert diff,, 3.3 KM, which will likely be done in well under 2 min. - I'd expect a full-on idiot on 170's to get down this in, maybe, 8 min? with about a 20% chance of survival ? considering the dozens of terrain conditions, they're likley not going to survive the 1st real compression, and certainly not survive the 80-140 ft flight off the inevitable jump. SO 8 minutes might be very generous, if it's a relatively 'easy' course.
A downhill course, even though ti should be well prepared for speed, is NOT a 2 mile Blue Groomer...
go see a downhill in person...
delusions of grandeur?
bet over a drink?