Do you intend to keep the 15 minute mashout?
In my experience (almost always brewing ales of >5.5% ABV from OGs of >1.050) the mashout is not adding anything.
As for mash time and boil time, I have posted a link* to a bunch of charts that include one showing that you do indeed get more fermentable sugars from the grist if mashing for 90 minutes compared to 60 minutes. The curve flattens out and >90 min is not the same.
If you mash for 60 min and a) increase the grain weight or b) accept the lower amount of extractables from the unchanged grain weight that's OK as your choice. Those who strive for maximum 'brewhouse efficiency' make their comparisons among their consistent methods - if they mashed for 90 min and got higher efficiency, would they change???? I don't worry about that. If time is more important than 0.004 in a specific gravity reading, then go for 60 min.
As for boil time, it is probably more about the evaporation total than the total time. True, there are some heat related / flavor-related happenings, but I have no idea if, in your recipe, they are finished by 60 min, or keep going through to 90 min. The rates may vary with the increasing concentration. How about boiling more vigorously to [hit a volume target sooner] rather than shorten boil time to [a specific number on a clock and take whatever evaporation you got]? Would it taste different? I do not know and I do not brew the same recipe again close enough in time to make fair comparisons.
To sum up = 1) mash longer than 60, shorter than 90 if you must, 2) eliminate the mashout 15 min, 3) boil more vigorously until you have evaporated the right amount and the boil will take less than 89 min.
* See the chart in the middle of the second row at http://bit.ly/2pnFnSH