49km or 26.5 knots is pretty good speed on a GO 151. Not many sailors with good skills and a GPS will be on a GO, so it may be difficult to get many responses. Many think the GO isn't a performance board, but that really isn't so true. They are a little heavy for durability, but the design is excellent for a pretty high level of performance.
A buddy of mine still has an old GO from 2005?? and a 133 hypersonic that he uses to cover winds from 10 to 25 knots. We were out together on Sunday, he was on his go and an 11.0 and I was on my Formula 160 and an 11.0. I was clearly faster than him and topped out at 23.2 knots, but the winds were only 5-13 knots.
The newer go boards should perform much better than those from several years back and they clearly can be fast, just a lot more mass to move through the water than the formulas, futuras or iSonics.
In the right conditions and the right sail, I suspect that you haven't reached your top speed yet. A smaller fin, and flat water will not doubt help a bit. Your body weight is the key to making a big board and sail go fast. I suspect that your body weight is over 85kg and if it is in the 90kg range, you have even more potential to improve your speed. I am only 77kg, and I have found that I am just not heavy enough to go really fast. You should be running about 120 degrees off the wind for max speed. Mast foot about 2/3 back from the front of the track.
Keep it up, it's fun to check your speeds on every outing, I have been doing it for over 6 years.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.