I haven't gone through all the
results here but I guess that '96 was the last time a biplane competed - Robert Armstrong in an S-1-11B in 31st place with a score of 78.64%. Lowest ranking US pilot was Ellen Dean in an Extra 300S at 34th with 77.09%. Highest ranking US pilot was Patty Wagstaff at 12th with 81.04% in an Extra 300S. Robert hasn't commented (that I'm aware of) whether he might've done better in a monoplane at that time.
Worth noting that the late Randy Gagne had a lot of fun with an S-2S in 77th place. Minoru Higa flew an S-2B into 78th place to beat an Extra 300S in last place,