Deltas/flying wings are very efficient.....in a strait line. They do scrub off a lot of speed in a tight turn, such as a race course. But if you can come out of a dive to kick in some extra speed to make a speed pass more impressive than just about anything, go with a delta.
Pulling out of a dive they lose speed too, nothing straight about it.
There have been a number of reports of F3d planes hitting around 220 with a slight dive.