I'm getting the carbon build-up blasted out of my Clubman S today. (43k miles. And a Countryman loaner!)
I'd like to hear any pros or cons of using a regular Seafoam fuel treatment or adding a catch can.
(I was interested in the simple task of adding the Seafoam into the fuel tank, not the more involved procedure of adding it directly into the engine.)
I'm leaning towards the catch can solution. Seems like an easy install.

Seafoam in the fuel tank will have no impact on the carbon build-up. It is only the PCV method that may have an impact.

Neither will help carbon build up technically...
The whole reason a direct injection engine has carbon build up is because the fuel is directly injected, instead of travelling thorugh the intake valves, it is directly injected in the combustion chamber. In a traditional engine the fuel would be "cleaning" the intake valves while the engine is running, which prevents build up of carbon. Thus, putting seaform into your fuel will not help clean the intake valves.... becase it would never travel through them. Thats why seaform treatments are done by injecting directly into the pcv valve, or the intake tract.

Also, oil catch cans may help reduce some carbon build up by catching blow by oil, but its not going to prevent carbon build up.

One option to prevent carbon build up would be injecting water/methonal into your intake tract. Get a hold of Jan @ RMW for this, as there is currently a group buy going on right now if you PM him.

But what are you driving habit like, first?
Do you just have a 5-minute drive to work, where the car barely warms up?
Or once the engine is warm, do you ever see high revolutions (RPMs)?

Our local Mini tech has been seeing cars with carbon build-up due to "granny driving"... no joke. It's been specifically noted in BMW's (Mini's parent company) owners manual for direct injected engines, if their vehicles are driven in short distances or low RPMs, its RECOMMENDED to go out and drive the car at above 4000 RPMs for about 5-10 minutes to remove any build-up on the valves. The heat of combustion and intake velocity should remove a majority amount during driving.

Now, I have a mild commute (~20mins) and the car sees red-line after the engine has properly gotten up to running temp and I haven't seen much of the carbon build-up others have photographed and documented. But, I do have a oil catch can and I do remove a decent amount of oil from the PCV system.

Its up to your discretion to determine whether its a driving habit issue (or lack there of), or oil-blow-by from the PCV system. But take into consideration many have proven that the oil vapor (not water condensation, which is due to environment like dew point and relative humidity) collection is true and is no doubt a contributing factor. The key is, how much of a factor - that's still be undetermined.

- Erik

Erik,
Thanks for your input.
I have a Los Angeles commute: 20 miles in 45 minutes.
So I'll just have to make time to get on Angeles Crest Highway and get the revs up! And I've order my catch can.
Do you get the RPM's above 4000 every day?

Oh yes, very much so!

- Erik

I am seriously considering water/meth injection as the only real solution to the problem, but concerned about how this might affect my warranty. I removed my catch can prior to my last service (10 min job ), so it wouldn't be an issue, but that would be a real pita with an injection setup. Comments?

That's a gray area for you to check on; talk with you dealer.
If you're a owner that cares for more of the warranty claims or possible claims down the road, I'd avoid it.
But if you're willing to take responsibility and willing to do some of the labor of your own maintenance / performance items, go for it.

- Erik