To get more power from an engine, you need to get more fuel and air in. A 52mm TB will in theory do this BUT as well as putting more air in, you also need to have a way of getting rid of the extra exhaust gasses as well.
Just changing the TB for a bigger model to get more power just dosent happen, it has to be coupled with a decent induction kit AND a good sports exhaust. To really get the best, a remap of the ECU is in order to take proper advantage of the extra airflow and o we go on from there. You can carry on then with bigger valves to get more charge in, high lift cams and vernier pullies. Bigger injectors with a higher pressure fuel rail and pump etc etc etc.
The thing you have to look at is each mod will affect other mods eg. a gas flowed head with polished ports is not much use with restricted air flow so you would have to look at the restrictions each side... air intake and exhaust, so one will make a difference but coupled with others will make a bigger diference.
Out of the ones you have stated it has to be the ECU remap.
All engines are designed for a maximum power output. This is running on optimum fuel with correct, clean oil and treated relatively gently. The avergae owner will run the car on cheap supermarket fuel, maybe use cheaper supermarket oil and not carry the changes out as often as they should plus they may be less than gentle with the engine. So to take this abuse into consideration and to build in some reliabilty they de-tune the engine ojn the mapping.
A good remap can put the engine back up to its original design spec. You will need to use Optimax or one of the higher RON fuels, you will need to have it serviced more often. You will get less miles to the gallon and suffer faster wear. Its the price to pay for increased performance.
Cheapest MOD (but proabaly thre least increase in power) is to change the air filter. It will only really be a benefit though is the exhaust is changed as well. Conversely an exhaust change alone will nto reap as much of an increase as if it were done along with an intake improvement.
If you are shoving more air in, you need a way to get it out faster, also if you have the ability to get the spent exhaust out, but are restricting the airflow in then there is little point.
A 52mm will give much better throttle response, I have one. If you wanted to go the full whack on the inlet side then Jenvy do direct to head throttle bodies for £400, you will need a new ecu to take them. EMerald ECU's are £500 + you will need £200 to map it.
You need a good exhaust and filter first though to see the overall increase. A ported head and new cam's again work well but you need the other induction / exhaust mods to see the full effect. A good tuning website for a full guide is at:
I brought a 52mm TB not so long back. Don't be fooled into thinking it will make the car go faster cus it plain won't, however racing the same car off the lights IE 105 vs 105 you'll see the difference as me and my mate tried as he fancied a 52 TB too, it didn't leave it standing like nos style but edged passed easy. Mainly sods off flat spots, pulls better up hills and accelerates up rev range quicker. If you want to do it for a nice even setup change your back box to and xpower one or 160 twin outlet one and the inter section for a 160 one too.