At least one s1 owner did do this with DMS. I toyed with it on the s1, but the reality was that you potentially traded a flatter torque curve for a few BHP at the top end. In reality this probably isn't going to make it a faster road car unless you drive everywhere at 6k rpm.
Air box and exhaust will liberate 5-10bhp if you have done that yet.
After that the key is to get the headders upgraded , airflow is the requirement to start with given the way the engine is packaged (so I was told).
There are some detailed threads on this topic,,
Hartage have a kit to upgrade the engine to 400bhp, but it was 18k plus labour back then...
