I'd go for a 2/2.5" system with a CAT in the location of the standard second one. One K11 owner I know uses a gtir CAT on a 2.5" system (with appropriate engine management) and passes his MOT no problem.
For info, a turbo will almost always IMPROVE/REDUCE emissions, not make them worse. A too lean mixture is not always good for emissions.
Also I am unsure how a "top mount" turbo would allow you to use the standard first cat? And i personally wouldn't want to remove a turbo system at MOT time!!