I am havin great luck with an OS#7 in the force .28 in my 9.5 buggy. Running a rb 063 pipe on the stock header and the bottom to mid range is incredible.
You can find used pipes on eBay all day long. I have an rb 063 and 9886 for tight/open tracks. I think I'm into both for $30. Picked up another 9863 (same pipe) with header for my budged truggy build the .28 is headed for, $12 shipped in the original box.
Going with a dedicated .21 buggy engine in the 9.5
Vintage R/C'r living in a modern world