The polling bump never lasts more than a couple days so mid-summer vs. late summer is irrelevant on that count. The Democratic 2012 convention was in early September, 2 months before Election Day. To help things out, the 47% tape was 2 weeks after that. 2-3 weeks later, Romney was leading. And in 2008, when the RNC went last, in early September, McCain came out leading Obama. 2 months later he lost by 7.
The purpose of the earlier convention is to cut way down on the number of Republican debates so as to (a) keep the country from seeing the GOP field fall over themselves to out-conservative each other (the unanimously-passed CNN and NBC boycott ostensibly over Hillary is actually for the same purpose) and (b) reduce the free airtime for guys like Ted Cruz and Rand Paul which somewhat levels the playing field against better-funded candidates. In a nutshell, party leaders fear losing control of the party. Or losing even more control.
The debates are all held before the primaries even start (and so won't be affected) and guys like Cruz and Paul will still get airtime (especially considering they're both very likely candidates). If you'd gone through and read the thread, you'd know the point of earlier conventions was so that general-election money could be used earlier.
