Originally we didn't base it on anything. The numbers were already floating around the Internet back when we first started the site. Don't know where they originated from, probably from one the early vinyl models that provided basic stats on the side of the box, but so far they seem to fit and so we still use them.
Official data files really only state the speeds of zoanoids, but we know from them that hyper class can go 45 MPH and faster. Some are rated at 90 MPH, though Thancrus isn't specified he was designed for speed and during X-Day was seen assassinating someone and moving so fast that the crowd only saw a blur.
The Guyver is faster than Thancrus and we have a scene in the Manga where G1 runs right past a moving car that spun several times before stopping, indicating the car had been going fairly fast as well. Also the scene just before it shows G1 shooting forward even as the wake of the blast field could still be seen knocking down a sidewalk tree but G1 was already half way down the block before the tree was even halfway falling down to the ground... Indicating an acceleration greater than 1 G.
A cheetah has 1 G of acceleration for comparison and can go 0-60 MPH in just 2.5 seconds.
So overall I'd say the 250 MPH top speed is a good ballpark figure.
Now for the flying, we don't have as much to compare to but we do have scenes where the Guyver seemed to just vanish and we know they flew away, but only the Gigantic has actually broken the sound barrier, so this too seems to match up to 300 MPH figure.
The momentum enhancers stated rating of 10x also suggests this if you take 1G acceleration for 1 second and multiply it by 10 gives you about 218 MPH, the Guyver can probably push that a bit past 1 second of acceleration, which brings it to about 300 MPH. Assuming of course wind resistance as a stop gap from further acceleration.