The link you posted didn't work for me but trying a few obvious others I found this one.
From my own testing I am sceptical of the gains they claim with only changing the snorkel. I have found better results by having a second hole in the lower airbox feeding cold air.
I have some pictures of some different Ford snorkels as well as the modified airbox I'm using and will get them on a site as soon as I can scan them and figure out how to create a simple web page.
My findings are based on using a Dwyer Magnahelic differential pressure gauge measuring negative air pressure upstream of the snorkel/airbox. I got the incentive from a 5 part article Autospeed magazine ran some time ago. If anyone is interested in doing some testing on different intake setups I am happy to assist with my gauge/time.
I have done a lot of research, (reading - heaps of American sites for cold air mods mostly on Hondas but a bit on Mustangs) on these so called 'cold air intakes.' I'm also a bit sceptical with just changing the snorkel &/or its size so I've also done what you have Kon - dropped a hole through the bottom of the airbox, used a K&N Panel filter & run a 3inch flexible duct from a hole in my front spoiler through to the airbox. I don't have access to air measuring equipment & I'm not really interested in them any way as actually driving the car is the best measure of 'success' - pleased to say that the car feels stronger up to redline, runs smoother, smoother through gear changes(manual), as for economy haven't had it on long enough yet. All in all a worthwhile mod, quick (half an hours work - after a panel beater cut the hole for me) & cheap, less than $40 using proper race craft duct hosing ($18/foot - you'll need 2 to be safe.) I'm also in the middle of setting up a web site, but can email some info & images to anyone interested, just let us know where.
I have priced the SSinduction set up after seeing similar at Summernats. They claim to be twice the size of standard though at around $400 for the kit (black plastic, more for colour code) I'm now very interested in your pics RAT.