I think the answer is "as fast as you dare to take it" .... the 1 litre is a little under geared, and that's on the same box with only a slightly more frantic final drive (4.27 vs 4.06).. the 1.3 definately so which is why the 8P is the box of choice (regular 4spd but +20% overdrive 5th).
You'll quite readily come out of the top of the power band in 4th and then it's just a matter of how deep into the red zone you can go before either hitting equilibrium (power falls enough that it wont accelerate) or you chicken out for the sake of the motor

(apparently you come out of the top end of power so easy, that for once the O/D 5th actually gives you more speed, even though you can't pull to peak revs in it so easily.. you'd get there about 105mph)
If you had a 6500rpm rev limiter fitted to it, then you'd bang up against that somewhere around 115-120mph (110 if you have the 1.0 diff) depending on tyres - VW recco max sustained revs is 6300.