| Those style intercoolers require exit ducting, enhanced capture aperture, and thermal barriers, in order to perform any better than stock; also, their increased performance is limited to a narrow operating range which doesn’t include road racing.
The stock intercooler performs very well in a variety of environments. The GP IC performs better in conditions where there are adequate recovery times. That performance advantage comes from a well designed IC cover and under hood duct interface which does a very good job of cooling its additional rows.
Notice there are proponents of a particular aftermarket IC which have that product for sale. A clarification for one of those seller's statement about pressure drop; pressure drop is a bad thing unless it is offset by a much greater increase in density. Pressure drop is a bad thing when the cause is the result of being “slowed down” by obstructions. “Bigger” is not “better” if “smaller” has a greater efficiency percentage. That well spoken about aftermarket IC has repeatedly been shown to perform at least as well as OEM; if you have disposable income and a forgiving budget, get it for bling factor or perhaps the marginal cooling ability, but don’t expect miracles. Once again the money thing if that matters, for the cost involved stick with the OEM IC and direct funds towards changes that make a greater difference.
I’ve had the OEM pulley, 15% & 19% reduction pulleys, and currently have an 11.5%, plus more than a few different states of tune. I have a sophisticated diagnostic/logging program with which I’ve monitored those configurations, so there are data points to go along with my subjective observations. I’ve also logged other MINIs and can use that data for comparative evaluations. None of those size pulleys require a larger A/A intercooler (37C and higher ambient temps are common here); get an aftermarket TMIC for any other reason than “you have to”. Additional fueling is a good idea in the 6k rpm upwards region while using a 19%, so 380cc injectors with proper scaling could be, and was for me, beneficial. The 380s are a prudent addition when the OEM rev limiter is raised and/or improvements in volumetric efficiency are made, using any size pulley. I have seen other public data which would indicate larger than 380s are warranted when a 15% or smaller pulley is used on a highly VE improved motor with improved exhaust system in conjunction with a raised rev limiter. In my case 380s are and were OK & safe up to 7k rpm.
I know the questioning posters want simple sound byte answers, but the reality is more like “it depends” with many attached qualifiers. The term drivability is subjective, when I think of drivability throttle response comes to mind first, and a smaller pulley has a minor effect on that behavior compared to other potential modifications. Yes a 19% will provide noticeable, earlier improvements in real world torque, but so can a tune, and the torque effect can also be emulated by modifying targeted areas of the ECU’s program. Head work and exhaust improvements were major contributors to my power gains over stock baseline across the board; a much higher percentage than known increases attributable to a pulley reduction. For the perceived torque effect, the Sprint Booster device did more for low and midrange throttle response than any reduction in pulley size.
There is much myth surrounding the 19% and many willing to perpetuate those myths. The increased high rpm heat created by a 19% is less devastating than most advocate. Accommodations can be made to increase performance potential in deference to the higher heat generated by the 19% in the upper rpm range, however, in my opinion; the 19% is a performance option past its prime. It served me very well for a long time, with no evident damage, but there are other options currently available which can provide similar benefit. If you’ve already made the decision to go 19%, or already have it, enjoy.  | 
