I can't say for sure as I have only ever had beetles but for every person who has a 'hot' bus there must be at least 10 people who run their engines stock - most of which I'm sure are happy(?!) I'm thinking that a stock 1600 with a well tuned carb/valves/timing setup will do 60 MPH depending on the final ratios. I know that the 'GT' gearbox had a bit of a longer final gear but I can't remember if that was IRS or not... I would have thought so though. They are worth getting if you can pony up the dough, and can find one! (in my very humble opinion)
I believe the list of T1/T2 engines go like this:
1200 (stock VW)
1300 single or twin port (stock VW)
1500 single port (stock VW)
1600 single or twin port (stock VW)
1641 kit to use with single/twin port
1800 71 mm crank 1600 setup
2000 73 mm crank 1600 setup OR normal 69 mm crank and larger barrels
2100 which has new barrels and crank in a machined 1300/1500/1600 case
Then you look at your cams and rocker profiles
I'm sure there are one or two more displacement configurations!
I know you can get a good amount of pull from a 1600 - most of these engines exist because they can! I'm not sure how necessary (or safe with standard brakes/suspension) the top end setups are for casual users