The NDP would probably be a bit more to the left, yes.
As for how Topp's leadership would look like, it's worth keeping in mind that, at the time,
theoretical polls had him performing significantly worse than Mulcair, with the NDP falling into third-place nationwide and tanking (comparatively) in Quebec. I'm not saying that Topp wouldn't be able to recover - indeed, Mulcair had been in third-place for most of the last few months, and look at where he is now - but the fact that Topp performs 20-points worse than Mulcair in Quebec and falls to third-place behind the Conservatives and the pre-Trudeaumania Liberals indicates he may well be in trouble for the first few months of his leadership.
That said, I think this produces too many butterflies to be able to determine how the 2015 election would shape out. For instance, if the Liberals end up returning to second, would Trudeau still feel it's necessary to run for the leadership, or would Bob Rae find the permanent leadership a more attractive job and decide to enter the race, as many expected IOTL? If Topp follows through on his promise to seek a seat in the House of Commons by asking one of the party's Quebec MPs to resign, will he actually be able to win given that the NDP may have sunk significantly in the province while the Bloc may have experienced a rebound? And, if the Bloc has in fact returned to their former levels of support, will Daniel Paille decide to stay on as leader, or in the event that he does will someone else replace him instead of Mario Beaulieu? Frankly, I think the political landscape would just change so significantly with Topp as leader instead of Mulcair that I think it's too hard to predict what would happen.
All that said, assuming everything goes relatively similarly to OTL, with all of the other party leaders being the same as OTL, I imagine the NDP is probably doing worse overall. Topp, due to his Western connections (involvement in the BC NDP, his former role of Chief of Staff to Roy Romanow, etc.) may have more appeal west of Ontario than Mulcair (given that he's from Quebec, as well as some of the more negative remarks he's made about things like pipelines and the oil industry), though I'm almost certain the party will be doing far worse in Quebec (with the prime benefactors likely being the Liberals and the Bloc).