TL;DR I had a new tensionsioner shock ($25 + $5 shipping from Way Motor Works) put in because it felt like my belt was slipping. It was. A lot. Completely changed my engine's behavior and powerband for the better, going from 200 whp to 277. Sound investment and easy install.

Full story: Been slowly working on my R53 for over a year, buying parts and sometimes installing them myself and sometimes having a shop do it. (See my signature for a full list.) Finally went ahead and had a shop put in the cam, injectors, and super damper and brought it in for a tune. Tuners had a heck of a time getting it right for some reason but eventually got it back to me with a 200 whp dyno graph, which I felt was okay but also kinda low for the money I had spent on parts.

Drove the car for a month and it just never felt right. Tuners had recommended I put in a larger intercooler so I ordered the Airtec one through Detroit Tuned. I also felt like my belt was slipping so I ordered the tensioner shock from WMW just in case. Once I had both parts, I brought the car back to the tuner for installs and a retune.

Few days later I get a call from the tuner telling me I'm not going to believe him. He said that apparently I had been running WITHOUT a tensioner shock for the past month. Apparently the shop that installed my cam etc. had mistaken my ALTA fixed tensioner for a "tensioner stop" and had removed it since it was rattling so much. But they never put a new shock in to replace it. The car still ran without it, just not nearly as well. When they told me about removing the so-called "tensioner stop" I didn't bat an eye since it was about 4+ years ago that I put that ALTA part in and had forgotten exactly what it was. (So I am putting that mistake half on them and half on me, for not knowing my car.)

Either way, once the tuner put in the tensioner shock and the bigger, blacker intercooler they threw it back on the dyno. Using the same maps they had built before with some very minor tweaks, they got 75+ more wheel HP and 55+ more torque over three consecutive passes. HUGE difference. And you can completely feel it on the road. Car feels like a monster when you get above 4k rpms and revs straight through to about 7000, screaming all the while. Lots of fun.

The take-away: If you have an older, high-mileage MINI (mine's at 125k miles) and feel like your belt might be slipping AT ALL -- go buy a new shock. It costs $30 and apparently takes about an hour to put in through the passenger side wheel well. Completely and totally worth it.

Also: whenever possible, turn your own wrenches. I can't always find the time due to an odd work schedule, but I'd like to think I would NOT have made the mistake of removing and not replacing a tensioner, thinking it was only a "tensioner stop." In fairness to that shop though, they have been excellent with everything else I've ever brought to them and I would still go back.

 

Interesting. I bought the whole tensioner, live and learn

https://www.waymotorworks.com/oem-belt-tensioner.html

 

Spray -

Just for the hell of it, put the OEM intercooler back on and give the dyno another try.
The reason I say this...the stock intercooler has streamlined dividers to "smoothly" direct the air into the tubes.
None of the aftermarket intercoolers, "that I've seen" have these flow directors. If you look into the interior of the core, you'll see, a lot of solid, 90 degree walls that the air slams into before finding its way into the tubes. A lot of disturbed airflow.

The stock intercooler, has the flow directors that allow a MUCH smoother flow of air...BOTH in AND out of the core..! I'd bet that there's even a positive difference in "manifold" pressure. And...the more air flow (rpm), the more a smooth flow matters.

Mike