Diminished hop utilization is not directly caused by higher gravities but moreso by the increased hot break matter onto which the iso-alphas tend to cling.
The conventional wisdom has been that late extract additions would create lower gravities and would therby increase utilization. However, this is dependent on the amount of hot break (not to mention, the rigor of the boil, freshness of the hops, etc.) and not the gravity alone. So although more iso-alphas would be in the boiling wort in a lower gravity wort with less hot break, the problem is that the late extract addition still just adds more hot break onto which the alpha acid can cling at the end. Because of this, I would argue that the late extract addition does not increase hop utilization as much as conventional wisdom now states.
Whether you get all the hot break early or get half of it early and half of it late, your hops always contact the same amount of hot break material. Duration of time with the break likely makes some difference, but I doubt it is as much as homebrewers tend to think.
To directly answer your question, the only reliable consensus on this issue was the general wisdom which has since been shown to be wrong (i.e. high gravity causes low utilization). Given what we "know" now about hop utilization though, I would definitely argue that late extract addition shouldn't bump your hop utilization as much as hop utilization calculators might predict. However, the full wort boil probably increases utilization more due to a lower ratio of overall hot break to wort. Since homebrewers have so long been assuming that what mattered was the gravity itself, I have still not seen anything particularly definitive on this specific issue. And since so much more goes into perceived bitterness than just isomerized alpha acids, I just don't think empirical observations adequately answer the question.
Doing a full boil will likely increase your utilization some, and late extract addition may increase it a bit more, but there is no definitive equation as to how much that boost will be since there are simply too many variables and a surprising gap of knowledge about how hop utilization changes. For the sake of good beer, you are probably wise to hold out some bittering hops and save them for late additions -- but I say that mostly because I'm a fan of hopbursting.