Not higher mash temp...higher gravity...Any wort that's using more of a given single malt will be darker than one using less of it. A mash and boil that gives 1.040 gravity using Pale ale malt will have a color that's somewhat lighter than a mash and boil that yields the same volume of 1.070 gravity.
As concerns the malty vs dry question, just try mashing a Pale Ale malt at a lower temp for more fermentable sugars. Try an A/B experiment with single malt beer using your Maris Otter type malt or something similar and compare a 154 degree mash to a step mash of 148 and 156 or even a single infusion at 150. That'll tell you a lot about what you can do without sugars. The use of adjunct sugars may be as much about boosting gravity in Imperials or DIPAs.
Your notion of using sugars may be a fine way to do things and may be the simplest approach, especially if you have a system that doesn't allow close temp control in mashing. You're going to make a good beer any you go about it, I'm sure.