Hi Timpo
had the tt foam one for a while i cant remember if it was in stock tho, the prefilter i didnt buy in the end as it replacesthe top cover of the air box, i know others have done that without issue to get more air flow but am not comfortable with it. what i might do as suggested to me was to put a bit of exhaust padding in the air intake to stop some of the bulkier debris getting to the filter
Cheers