McGrath and Srinath are essentially the same age (Srinath is 5 months older) and I'd say it's fair to say that at equal points in their career Srinath was always faster. Unfortunately, both were the best part of 30 by the time speedguns were perfected (the first time both bowled in front of one was the Super Six of the 1999 World Cup). 27-29 is generally the sort of time when a bowler can expect to be at his peak pace-wise, and some can maintain it for longer. In the 1999 World Cup Srinath was comfortably the second-fastest bowler on view after Shoaib Akhtar; McGrath's fastest balls were certainly quick, but he was consistently in the mid-80s while Srinath went over 90 almost as often as not. How they compared in the 1995-1998 period will never really be known. McGrath was not, I don't think, thought to be that quick when he very first emerged; Srinath, UIMM, was. This is not neccessarily reliable - as Manee notes, the speed a batsman thinks a bowler is bowling at and the speed he is are not always the same thing. I imagine McGrath probably lost more pace off the pitch than Srinath (in fact I know that the fact that he lost a decent bit off the pitch helped him catch rather than miss the edge more often than so many) so even though he was better he may have seemed slower even if they were actually releasing at similar speeds.
So in the 1999 World Cup, McGrath was certainly fast enough to classify as rather than fast-medium, but only in the lower ranks of fast; Srinath was fast and very fast. Srinath was still bowling in the 140s in the early-2000s; McGrath was not (IIRR he was actually bowling slower than Greg Blewett - who was far quicker than his run-up suggested - at one point). But McGrath lasted until 2006/07, while Srinath was basically done by 2002. Even though Srinath could bowl quicker than McGrath, he was not able to bowl for anywhere near so long.