The unbeaten League One leaders are just two victories away from clinching the third-tier title after romping to a 24-point lead.
But the Light Blues have not faced top-flight opposition since losing 3-0 to Dundee United in last season's William Hill Scottish Cup.
They now face Albion Rovers in the quarter-finals of this year's competition knowing a victory will likely pair them with a Scottish Premiership side.
The bookies believe they are on course for the final and have priced them as the 3/1 second favourites.
But Ibrox boss McCoist is uncertain whether his team is ready to take on one of the nation's big outfits.
He said: "I'm not sure if we are ahead of schedule or behind.
"It's difficult to gauge with cups because you do need the luck of the draw.
"Should we be amongst the favourites? I don't know. Dundee United look not bad while Aberdeen have beaten Celtic twice in the last four weeks.
"I don't look at it and think we are second or third favourites because, with the greatest of respect, nobody really knows how to gauge our team.
"Cup ties are one-off games anyway but because we are not playing at the top level no one really knows where we are it, so it doesn't really bother me what the bookies say."
But McCoist is adamant his side will take no chances against the League Two Rovers, who have already knocked out Motherwell and Stenhousemuir from the divisions above them.
"Rangers have been knocked out of cups before when they were in the top flight by teams they should have beaten," said McCoist.
"It is no different in respect to Motherwell, who were favourites to beat Rovers when they played, as were Stenhousemuir. So that indicates that the favourites tag doesn't really matter.
"The fact that we are at home and have already beaten Rovers this season would indicate that we should be strong favourites. We just need to make sure that we deserve that tag by winning."
McCoist has never taken his team to the last four of a major domestic competition but, a massive upset apart, he should end that run on Sunday.
But the Gers manager insists a prolonged cup run is still secondary to the champions-elect's promotion aims.
He said: "Sunday is a massive game. But, as you would probably expect me to say, in the grand scheme of things it's not as important as winning the league.
"The fact of the matter is that to get back up to the top division as soon as we can (is the priority).
"But the game is important in that it's a cup game and a chance to get to a semi-final.
"That would be a real boost to our supporters and I would actively encourage them to come out on Sunday, as they have all season, and make Ibrox as hostile and as noisy as they possibly can."