Rhi, I know you are a contract installer, so you'll probably enjoy this -
I have Road Runner cable modem service through TIme-Warner Cable. I don't have cable TV, so a contract installer came out to get me connected. He got the cable installed and connected the modem, it synched up after a minute or two, then he scribbled a bunch of numbers on the work order and left. While I was helping him route the cable, and asking some basic technical questions (i.e., what frequency, levels, etc.) he said he didn't have a meter with him, but if the modem synched then we were OK.
So anyway, he just pencil whipped some numbers in the signal strength boxes on the work order. No big deal, I thought. Then I started having problems losing my connection, and a Time-Warner RF tech came out for a look-see. Soon, I noticed a very puzzled look on his face, so I asked what the problem was. Seems he had spent the last 1/2 hour trying to figure out why my return signal was a fraction of what was shown on the original installation. After I told him what the installer did, things made more sense. The problem turned out to be that the original 3Com "tailfin" modem died. He gave me a new Motorola and it's been working fine ever since.
The other point is I'm just verifying what you said, that if the modem synchs there is nothing you can do to increase the speed. It either works, or it doesn't.