In case you are wondering if this will work, it does.
Installed a 15% reduction pulley and 1 notch colder spark plugs several years ago.
JCW injectors yesterday. That's 340cc to 380cc.
No computer changes.
Pulls stronger and I no longer have the lean backfire when downshifting.
Should be a little better on hot days too.
One interesting note, after the installation was complete, it sounded like it was
running on 2 or 3 cylinders. Bad new injector? Checked resistance of injector,
voltage at plug, all good. Bent the injector spade connectors ever so slightly
toward the valve cover, reassembled, cleared codes and it was fine. So bad connection
at the plug.
These two items are good bang for the buck!
Other than that, powerflex bushings for trans, lower control arms.
Definitely helps torque steer.
Haven't gotten to the rear.

 

The pulley is doing its job and adding boost, delivering more power throughout the rev range. Best bang for the buck.

To take advantage of the increased capacity of the new injectors, you’ll also need a tune. A CAI and less restrictive exhaust will help too. It’s all about getting more volume through the motor.

 

Welcome to the forum as a newer member, although clearly not a new R53 owner!

As such, perhaps you've not viewed my (sig) thread topic. I'm still in discovery mode how my engine was modded for performance when new (noting claimed hp) but suspect it has larger injectors and colder plugs and likely cam and/or cylinder head work in addition to what is visible externally. I do know that when I start the car cold, it'll blubber a bit. Can't just start cold and drive off. Once warmed, runs cleanly with smooth power to the rev limit with characteristics of highly tuned engine.

Point mentioning is a question to others with modded W11 engines, whether larger than OEM spec injectors typically result in cold start running as described?

 

So what is the question? Not that I'm going to answer it for you, but maybe someone else can / will or is prepared to help you out.

 

If we are talking 380cc, no, I wouldn't expect it would. They were standard in JCW cars and nobody I know of complains of cold starts.

Your car has a lot of unknowns. It could be that you have even larger injectors, modified cam, we just don't know. While I envy your car for the power and the mods, it would be difficult to figure out what is what and how it was modified. It surely has a tune, but what was it tuned for, drivability, or track use? I'm guessing track use by the original exhaust setup, but who knows. All I can say with certainty is you have one hell of a car.

 

You can install 3800 CC John Cooper works injectors without a tune and the ECU will calibrate due to the tiny upgrade of the injectors and there's an a bit of calibration leeway in the ECU iranice to see you I ran him for a year both untuned and tuned with 0 issues is 0 issues what
With the cold start issue I would check if you're if you're 02 sensors are functioning correctly or if your map sensors are clean

 

You can verify by checking the fuel trims, if they stay within -10/+10 you are "ok", but its way better to stay around -3/+3

 

@smschriefer - Agreed on all mentioned aspects. I'm out of town at seasonal 2nd home a few more weeks but will check plugs and injectors for ID markings. Best to start with engine analyzer measuring current fuel-to-air mixture. I won't change anything without specific data but depending, I'll consider mildly de-tuning for strictly street use, i.e. injectors no larger than JCW spec and plugs to match.

@MiniManAdam - Add'l info on those injectors also good to know. No codes showing whatsoever, so suspect the sensors are fine. The OP had it smog checked morning of my purchase (not realizing wasn't required for registration where I live) and passed without issues. I have that report and will check the readings when I get back home.

@nd-photo.nl - More very useful data!

@Bruce.K - Just now read your intro post from 5 months ago which was light on details plus no pics. Seems you're keeping the R53 vs buying a Countryman?

 

I figured these two mods would probably be the most you could do without exhaust, tuning etc.
Basically a budget upgrade. Does ok! Get you a ticket for sure.

 

Since you bought your car in May from the original owner (with only 4k on it), did he not tell you what mods he had done to it? Or did he not know?