It's not about the rpm's, it's about the load on the OD lockup clutch.
You can use OD, but manually engage it once you're up to speed, as soon as you get below 100, press the button to disengage the lockup.
FYI, the S1 drops ot of OD really easily if you're below about 120, this kind of behavior can cause excessive wear with a significant load.
Fitting a bigger trans cooler should be a no-brainer - if you have an auto-parts store or wreckers, you'll find something suitable.
She'll cope fine - where are you moving to?