Good idea. Sorry I can only contribute European suppliers, being from Europe and all that. I hope somebody here will find below info useful though. Most offer separate interesting aftermarket parts that you can have shipped for yourself.
: They make a very smooth & clean upgrade to 150/180hp for the small 120hp K-Series and a turbo version to 300hp, also for the K-Series.
: British company. Developed the 135hp stage II kit which went in to the Elise Sport 135. No doubt they can do something extra if you want.
: These guys sell parts for Elise only. You can shop cart and crank out some more power yourself. Heaps of light-weight alternatives for an Elise diet too. They did several custom conversions including one natural aspirated 205hp engine.
: Well known. Used to offer 190hp upgrade, now 230hp supercharged since the S2.
: Offer many parts for S1 and S2 as well as tuning upgrades. They explain some Rover upgrades on their site in detail and seem focused on driveable and entertaining power rather than upping your bragging rights - which is nice.
Bell & Colvilll
: Performance upgrades with style. Famous dealer in the UK. Also do very British style touches.
: Professional aftermarket parts for all Lotus types including of course Elise. I feel like ordering their silencer, sounds brilliant apparently.
EDIT: Added CNeil, Moto Build and Bell&Colvill