Hi folks - thought I would chime in as I spent the best part of 3 years while with my s1 looking at this.
Standard recaps by dms will give you a bhp benefit, but at the expense of lower down torque. So you might gain some bragging rights at the pub, but you'll likely lose out on the road in real terms.
The only real option is going for serious engine surgery, there were kits available to upgrade (German tuners mostly) to the alpina spec and beyond, but you're looking at boring out cylinders fitting new pistons machining crankshafts etc, which is all doable, but the cheapest quotes I saw back then were for around 20,000 gbp just for the kit of parts (lots of titanium happening there) and that would get an extra 75-100 Bhp at best. Bear in mind that you could do an engine swap but that you'll also need to swap the electrical loom, and that. Evans reconfiguring airbags, abs etc etc..
Yes the headers are restrictive, but there isn't a great deal you can do as the shape of the car somewhat dictates this..
After 3 years with the s1 I realised the cheapest upgrade was an s2. Likewise the best power upgrade for the s2 is an s4..
You can do upgrades, but they will cost a serious % of, the cars value and realistically you won't get it back...
The best upgrades I found in my time with my aero was actually tyres. The right rubber at the right pressures made a huge difference...