When it works for VW's in-house brands they benefit from the group's massive investment in engines, platforms and technology and are 'gifted' cars they'd no way be able to develop on their own. There are risks, however. Just ask those using certain diesel engines, for instance.
In the case of hot Seats, though, the results have generally been good. The new Leon Cupra I've just been driving has the power of a Golf R and costs about the same as a GTI while getting much of the kit VW charges extra for as standard. Seat is geographically and emotionally far enough away from Wolfsburg, that it seems to get the freedom to tune and calibrate the cars to be a little more expressive and emotional than their German-branded relatives too. Sadly, the new car seems pinched just a little too hard by group politics, VW seemingly keeping Seat in its box to avoid harming sales of its own product.
Inevitably this got me looking further back into the history of the hot Leon, inspired by Seat's decision to bring along a few examples from its historic fleet. I've always loved the first generation cars too. The platform underpinning them was Mk4 Golf, shared also with the first-generation Audi TT and the contemporary A3 among others. A solid start. The hot versions of the Leon got the Audi-built 1.8-litre five-valve engine shared with the S3 and TT, putting out 221bhp – seriously spicy for the time. Audi also donated the interior fixtures and fittings (hand-me-downs don't come much better) while Seat lifted the Teutonic sobriety just enough to make it look exciting. So it goes like stink and has a nice environment in which to enjoy the performance.