Better quality versions of the Scottish Water utility plan (showing the sewers only) that Toomse uploaded.
North end of Hope Street
Note the text annotation alongside the sewer under the footpath on the west side of Hope Street...
"MH 8904 upstream lies within a service tunnel"
Close up of text annotation
Judging by the info on the sewerage layout plan, it would seem this service tunnel isn't the one that's being suggested to exist in Hope Street as it appears that it is only 300mm wide by 900-1300mm high. It does seem to be in the exact same position as the "Corporation Tunnel" indicated on the schematics uploaded by Toomse though. The sewers in Glasgow were once owned/maintained by Glasgow Corporation so maybe the tunnel indicated on the schematic is just the small service tunnel?
The manhole that Toomse took a photo of outside McConnell's roller shutter door on Cowcaddens Road is in the exact same position as a sewer manhole shown on the sewerage layout plan - I think the obvious conclusion would be that it is a sewer manhole rather than an access manhole to a large tunnel.
Not saying some sort of large tunnel doesn't exist in Hope Street, just maybe that the "corporation tunnel" indicated on the schematic isn't related to it.