The only time I've been able to get 8000 out of mine is by pitching the stock impeller, which is a 10/21. That was 2 years ago now, but I'm pretty sure I actually lost 1 mph by doing it. Granted, I'm not a professional impeller bender haha.
Currently I'm running a Solas 13/18 with a 4" intake and a Fizzle F1000 and I'm still only spinning 7800 lightly loaded. With 2-3 adults it drops down the 7600-7700. 44-47 mph depending on the load and conditions.
I also have a Solas 14/19 and going by the numbers it should have a lower max rpm, but it doesn't. Honestly I can't tell the difference between the two. That said, I've never ran them back to back on the same day. Maybe I will on my next days off.
There is also a Solas 14/19R which is supposed to mean it's been placed in a pitch block to ensure accuracy. So how did they determine the pitch on the regular 14/19 or 13/18???? I don't have a pitch gauge, but on the bench my 13/18 and 14/19 look the same to my untrained eye.