splitters......... hmmmm indeed, most of them are alot of money to gamble with speed bumps, but bring the look of the front right down to the tarmac, plus if you buy a good one then it will assist with air channelling or 'ground effects'
the osir one for me is way too much money for so little impact, i think RR has the right idea, subtle and OEM looking but big enough to make a statement, suits the car perfectly. if i had a gti i would have already bought one of those, but i don't so i have to look elsewhere and there's just nothing out there worth looking at. choice for all cars can be limited as theres only 1 way to cook a chicken...ie theres only so many splitter designs that can be done before theres nothing new to make as it already exists
my new prototype splitter is almost finished now, the mould is in it's last stages so should see an actual unit in about a weeks time, I've got it so that it sits flush onto the bottom of the bumper angling down and under with a flat shelf to allow flat airflow going under the bumper for about 10 inches, as it gets towards the side it flares outwards and upwards a little to get up over the side spoilers and give some design to it. Innercore will give it the strength and double skinned too so i don't have to fear the speedbumps.
if you seek downforce then you need to ignore things like osir, caractre, reiger and the like as it's cosmetic. what you need to look towards is a flat shelf, such as the type held on with struts attached to your bumper for strength ala touring car:
then you would need to combine this with as much sheeting under the car as you can manage, flat is what you're trying to achive to aim for smooth air with as little turbulence as possible, as it slides under and past the car the air then needs to hit a diffuser to force it out in straight channels:
take closer look at some of the scoobys in your area, most of the guys who get hardcore with those utilise this sort of gear.
if you do shelf the underside you will then need to adress engine cooling ducting and brake ducting for air entry and exit channelling such as vented side skirts and vented front grills. then you would need to address the top rear of the car, huge spoilers would counteract the ground effects so you would need something adjustable and even accross the width
..... and that's my short answer