Hey
I have got an 03 Ts as well and i'm in the tuning game myself. The problems with this engine is that its highly tuned already. A NA 1.8 with 187hp is quite an achievement. I know that Lotus don't mess with the engines themselves and only change the ecu but that is because toyota keep there warranty so Lotus don't have to deal with it, also the ecu doesn't give any more power and only changes how the lift is initiated and the limit is upped to 8500rpm.
The exhaust and filter should make a difference but not enough to notice, especially at £500 for the both with only 5hp difference at most. (also the engine management compensates for the changes and after 500 miles the power is lost again)
You can change the top internals but again its big money for the little power gain. A supercharger is the way to go and lotus are taking these up to 260hp but the 220hp option will be the best. Compressions are low enough to not lessen the life expectancy and anything after 220hp will give you some crazy torque steer anyway so theres no point.
As you have the same model as mine you will notice that the car feels like its on stilts and the brakes are not good enough. Id start with 20-30mm lowering springs, 17" wheels lower profile tyres to sharpen steering and some bigger vented brakes. The fade on the standard pads and discs come quickly.
After that rant though, I love the car. Its definitely a wolf in sheep's clothing. O and if you are changing the springs and wheels tell me first and i can work out the maths for the numbers you want so the car behaves properly