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Based on how it's made I'd say it could be trimmed fairly easily and I'm pretty sure it would not impact it's strength at all, it's just a solid piece of plastic so making it shorter won't impact it as long as all of the mounting points are preserved.
Why trim it - are you concerned about nailing it when you go off track ?
Do you own a LS already? If you do I would keep it stock and get the WC splitter, I paid $425 for the kit, not bad and you can keep the stock one for when you sell the car as a collectible. You can trim it with a router if you want but by the time you DIY or pay someone to do it you might decide it would have been better to just buy the WC.
What is a WC splitter? I got my first look at a '12 LS this last weekend at Road America and that splitter scares me. Is it safe on the street or is iit going to get ripped off when I leave my driveway?
WC splitter is the Pirelli World Challenge splitter. Slightly smaller than the standard LS splitter to meet the rules for that series and also fo NASA American Iron and another SCCA series. Less chance of catching on driveway ramps or speed bumps and more downforce than stock splitter. It's also cheaper than buying the regular LS splitter by about $300.
You still have to be careful but I have run mine for over a year on the street and track with no damage.
I would advise getting it from your local ford racing dealer--online the prices may be cheaper but typically come with a $100+ freight shipping surcharge due to the size of the part.
You have to remove the fascia to install, but do not need to drill any holes.