Personally been using the Garmin Geko 201 together with a friend who has the same one.

We clocked similar speeds (16.8 knots) to yours without a spi on a Taipan 4.9. Remarkable the speeds on upwind, downwind and reaching legs were very similar. That was something we didn't really expect. F5 to high F5 on a lake as well but some chop build over time. We were to chicken the hoist the spi that time. The course was also relatively short because mono's were on the start line with us. The C-mark was only meters from the shore and I actually had to jump off 2 times at the c-mark to prevent beaching the cat. Made for some nerve racking C-mark roundings. The lake was an old Sandpit so the banks are very steep.

See attachment.

Note this was certainly not one of our better rides together. Was our first time together as well. Later plots show more consistant sailing but the plots are less well developped to show on the net.

GPS's are cool that is for sure.

Wouter