Here's a top speed calculator I found:
http://assassinracing.com/tech/horsepower_calc.shtml
The golf's hp is 140
cd is .304
area cd is 2.22 m2 (23.9 sq ft)
air resistance is .304 x 2.22= 0.675
weight is about 3200 lbs with a single driver.
According to the calculator, assuming those conditions in the test, you need about 150hp to go 140 mph. Tires, air temp and pressure, car and road condition can change all those.
Redline is 5000 so assuming a manual transmission, using my calculator the top speed limited by gearing is 187 mph. To go that fast you would need about 350 hp.