Hi Adam,
I have an Air cooled.  It was slow when I bought it in 1994, it was slow
after I put in a rebuilt Mark Stephen's engine.  I have had no problems
with MSHP but I hear they are out of biz.  It is still slow.  Face it,
it is going to be slow with only 56 HP.  No new stock engine will change
that.

I drove to Reno twice this summer.  I drove between 55 -65 trying to
keep it around 60.  I found that oil temp goes up quickly when you try
to push it (65 mph).  I finally had to kick back , drive slow and look
at the people being pulled over for speeding when I passed them.  I
don't get tickets.

Acceleration is slow especially up hill.  On I-80 there are some looong
hills.  I was in second to third a lot.

We killed our lawn this summer (Utah is second driest state in nation)
and tilled it in to install xeriscape, (low water use landscape,
desert).  we used a friends 8 HP roto tiller to till the yard.  As we
were cruising to the Burningman, we kept commenting that the HP of the
2.0L Air-cooled engine was only 56 HP equivalent to 7,  8 HP roto
tillers.  when put in that perspective, we were happy to be moving along
the earths surface at 60 MPH. Envision 7 rototillers pushing us and all
of our stuff up those long hills and across the desert.  Now its not soo
bad is it?.

good day,
Ken