PD130 Running Cold (60-80C)... can I still drive it hard?

[VAG2020]

I've been driving my dad's stock B5.5 Wagon (6 Speed Manual with the PD130 AVF engine). It gets driven maybe 200 miles a month right now (except for when I visit), and isn't really being looked after - its been parked outdoors for 17+ years and it's in pretty rough condition (but only has 118k miles on the clock).

The thermostat must be suck open, because it never seems to hit 90C. On highway driving it reaches 70-75C. Going up a steep hill in 2nd gear it might reach 85C.
I just replaced the CTS so i'm fairly sure the thermostat is the problem here. Our garage is closed for the holidays, so it's probably going to be another 2-3 weeks until i manage to get the thermostat replaced. (BTW this is in Italy, not the UK!)

My question is - is it harmful to push the car hard (flooring between 2-4k RPM) when it's only showing 60-80C ? (given that I've waited a good 15-20 mins for the engine to "warm" up).
So far all I noticed a slight increase in MPG which i'm not too bothered about.