Hard to put a figure on it, but Akram did score over 6,500 international runs, with 3 tons and 7 fifties in tests and 6 fifties in ODIs. I think it's fair to say that McGrath won no games with his batting, and would never have done even if he had played for a millenium.
Another way of looking at it is that the difference between Akram's test batting average (22.64) and McGrath's (7.36) is over 15 runs/innings, or 30/match. So any match in which a hypothetical Pakistan beat a hypothetical Australia by fewer than 30 runs could be chalked up to a win thanks to Wasim's batting. Of course, I would like to see a sample size of 10,000 matches before feeling confident in drawing such a conclusion.
Re: matchwinning bowling, it should be noted that McGrath has 3 10WMs in 124 matches, and 29 5WIs. From 20 fewer matches, Akram has 5 10WMs and 25 5WIs. This means that McGrath got a 10 wicket bag every 41 matches, compared to Wasim's 21.