Yes, checking the Comcast signal to a HDTV would be worth checking, as would playing a Blu-ray to the projector. The additional testing is also useful for convincing the call center that it isn't your equipment (unless, of course, you find out that it is).
These are probably "dumb questions" but sitting on this side of the screen I have no idea if these were double-checked:
Are you sure you have the Comcast STB tuned to a HD channel? They generally have one range of channels for SD and another range for HD, so those who are on an SD-only tier will get just the SD channels, but those on a HD tier will get both SD and HD. For example, where I am, 1-699 are SD and 700 and up are HD, so my local ABC affiliate comes in on Channel 2 (STB displayed channel) for SD, but for HD it is 702.
Are you using the highest quality connection between the Comcast STB and the projector? If both have HDMI, almost always HDMI would be the best choice. If no HDMI, component video (Y, Pr
, Pb RCA connectors, plus L & R audio RCA connectors) would typically be second best. Composite video (1 RCA video connector + L & R audo RCA connectors) would be next-to-last choice (last choice being NTSC analog RF coax and forcing the attached device to tune to NTSC channel 3 or 4). Both composite and RF coax are strictly SD and one or both types of connections are typically the only ones provided on a Comcast-issued STB for SD-only customers.
Is the STB configured to send a HD stream to the projector by whatever connection method you are using? On my STB, I have to turn the box off, then press Menu on the remote to get to the setup screen that allows me to configure what resolutions are allowed and whether or not to force the box to rescale all signals to a given resolution. While there, there might be a "sharpness" setting for analog connections so, if you are using component video, you might need to adjust that and see how various settings of appear on the screen.
Talking about scaling, if you are using the STB to rescale to a given resolution, is that your projector's native resolution? Almost always you want only one device doing the rescaling, either the projector (in my case, my TV), or the STB, but not both
. If the STB has a "NATIVE" setting so it passes the same resolution on that the channel it is tuned to is using, then the rescaling is left to the attached projector (or TV), and that is likely to do a better job rescaling; but if the STB is configured to rescale, if at all possible, set it to rescale to the TV's native mode.
Anyway, diagnostic screens and menu screens are usually presented in the same resolution as the channel the STB is tuned to (this might not be true with the configuration screens from powering off the STB and then hitting Menu), so if I hit the program guide button or a menu button while tuned to what I think is a HD channel, and the text is fuzzy, I would be looking at the settings of the STB and the connection to the projector.