There are a whole load of performance enhancements for sale on Ebay (Magnetics which are both Fuel Savers and Power Enhancers - WRONG) Fuel is covalently bonded and as a result cannot be magnetised. There are only 4 Elements that can be magnetised, 4! Fuel is not one of them.
Resistor chips, just ends up costing you more in fuel, and now the Electric Superchargers, and more resently the unpowered Supercharger Intakes. These claim impossible revelution speeds of the fans, claiming it increases the amount of air being forced into the engine. If anything they will actually block the air flow.
Some sums, a supercharger on an MR2 generates about 60bhp, however 30bhp is used to drive the supercharger, leaving an the remainder to drive the wheels, hence you have a net gain of 30bhp, maybe less. However the important point here is it takes 30bhp to drive the supercharger in order to generate the force to push the air into the engine. It generates 12-14psi, 1 bar which is a lot. 1 horse power is 756kw, that is also a lot, 30 horse power is more than 22kw, 11 kettles, or 25 Microwaves, all switched on MAX POWER at the same time. That is the energy needed to force the air into the intake. These electric superchargers have a supply of 12 Volts, consume 30 Amps (Claimed - I do not believe it the wire needed for this is getting to be quite a thick gauge) Go to B&Q look at what wire is needed for 30Amps, More than 2.5mm (23Amps) Less than 6mm (46Amps). Besides that 30Amps at 12Volts is 360 Watts, not even 1/2 a horse power, THAT IS NOTHING, it would take ten minutes to warm a cup of water. That is 60 TIMES LESS than the mechanical units, which is designed by the car manufacturers themselves.
SAVE yourself some money, forget these gimmicks. If you want better performance it is going to cost you, An exhaust can add 5-10 BHP or maybe nothing, Airfilters 3-5bhp MAYBE, an ICON tuning piggy back ECU (The real deal) may add 8-10bhp But in the mid rev range, nothing at low or high revs. Beyond that you need to get a specialist ECU, change your CAMS, Pistons etc etc. You have to spend real money for real gains.
Forget the too good to be true items you see on Ebay they are penny items being sold for lots of Money. If you want a Resistor Chip Mod get a 4.3k Resistor (Get the GOLD Banded ones as they have a tighter tolerance) Stick it inline with your Air Temperature Sensor - What your fuel gauge drop and your engine will get damaged. That is what you Buy for £5. The Magnetic - Nothing special about it, you want to do this mod buy a Neodybrium Iron Boron Magnetic (Strongest Rare Earth Magnet in the UNIVERSE) Strap it round your fuel line and feel the nodifference, you want an Electric Supercharger, get a PC Cooling fan and shove it in your intake, failing that just get a brick and shove that it, it will do the same job!