While they're not illegal here if they result in any increase in induction noise then it's an offence to have them fitted. This also voids my insurance and I know of several case where an insurance company has not paid out on a claim when the assessor noted foam filters on a road bike.
They also have a ruputation for catching fire if the engine backfires - I'd prefer not to have a fire directly underneath a tank containing 25 litres of 95.
The Ram Air setup also has a reputation for breaking / cracking. I hate to think what would happen with the force of a backfire.
I understand it's a personal choice but it's not one I'm comfortable with considering the negatives.
Here's a relavant TriumphRAT thread on the topic worth a read (Unifilters specifically but yes they are a foam filter)
WARNING!! UniFilters catch on fire!! - Triumph Forum: Triumph Rat Motorcycle Forums