...the ship indeed spends a lot of time at impulse, to let the engines "cool". Perhaps they do move at warp nine-plus a lot, but then pay the price, so their average speed is no better than warp six...
That would explain why they spend so much time at impulse, something that always bothered me about Voyager. Of course it was really done for budget reasons, since hanging the starfield curtain outside the set is a lot cheaper than matting a starfield in all the time. However, I can certainly buy your theory; thanks Timo.
Regarding the issue of Voyager's speed, I give you this quote from "Caretaker"
STADI: Intrepid class. Sustainable cruise velocity of warp factor nine point nine seven five...
The key word there is "sustainable". I suppose we could just say that Stadi got it wrong, or was exaggerating?