Yes a good exhaust and extractors will do the trick. But it all depends on what you have on there now. If you already have extractors then changing them to a different one will not give you that much more power (if any at all). If changing from standard mini exhaust manifold and pea shooter exhaust then you will get maybe 8hp(?) extra which is a lot if your stock 1100 has only got 40 odd hp.
I have not read this whole thread but to get more out of your mini you need to:
1) make it breath out better, extractors, exhaust
2) make it breath in better, intake manifold etc
3) give it more fuel and more air, bigger carb(s), more efficient air filter
Once these "bolt on goodies" are in you will get more out of the engine without serious surgical work.
Common surgical work:
1) Head work including bigger exhaust valves, larger ports etc
2) Bigger bore, like 1380 out of a 1275cc block etc
3) Running a hotter Cam, like a Graham Russel RE13
Speed through handling:
1) Bigger brakes, you can go faster if you can stop faster!
2) Lowered/tuned suspension to make the mini go around corners like a scalded cat on plush pile carpet
3) Good tyres like Yokahama A008's or similar
I confess I too am learning so this is only scratching the surface
