K & N or Pipercross Air Filter with Remus can?

[Falcon 269]

I wouldn't recommend changing the filter from stock unless you have had the carbs rejetted.  Although the Remus can and a less restrictive filter will work well at higher revs, you might find that the fuelling gets a little lean at part-throttle lower down the rev range.  Not lean enough to cause engine damage but it could induce lean surge at around 4k rpm.

Of the two filters you mentioned, I'd go with Pipercross.  K&N filters for the Gen 1 frequently don't fit correctly without trimming the top moulding.