I cannot fathom where you get the idea that a 1/3rd Metallic Ratio enhanced 7.8 Munsel Gray would be just .5 gain.
Universal Gray is 8.3, and by itself has been measured out at .85 gain.
Silver Screen (Behr) measures out at .8 Gain, and it's virtually the same shade...without any metallic content or Poly...as Veil.
The OP is Rolling...and my advice to stick with a lower metallic content supplemented with being thinned with Poly is both wise and well advised. If he was spraying, absolutely I would advise otherwise.
And somehow you have an idea that the reflectivity of the Silver Metallic is greater than the Pearl. That Sir, is dead wrong. The primary purpose of using Silver Metallic was to have the presence of both Mica and a darker Gray base. Omit the SM and under some circumstances, the SM's Gray base can be substituted by a Tint in the required White base, and the missing Mica replaced by upping the Pearlescent content.
Also...Silver Metallic...any Brand Silver Metallic (...except Liquitex...) has "Black Mica" within it's composition, and too often it manifests it's self as tiny Black Specks. Also, the discrepency between the Dark Base, the Bright Mica and Dark Mica can produce the undesirable trait of Graininess.
Most every aspect of the Formulas I present are tendered to reduce that deleterious effect. You need only make note of how far reduced the percentage of Silver Metallic has been in Silver Fire to understand that.
Oh...it's a great thing you and others have taken to using SM to up the ante....even those "enemies" you referred to...
It's better than not having it. But just like those others were hold-outs for so long against using "any" Mica-based Metallic, your rushing to dismiss valid findings that are backed by far more actual experience and End User results than you can dredge up.
White Pearl was / has been a mainstay of every Mix I have ever created since the first MississippiMud 1:1:1 mix hit the Forum Boards. It offers enhanced reflectivity without adding the attenuation that having a Gray base brings along.
If the OP does not need as aggressive a ALR paint that the Veil base will offer, he can either switch to Universal Gray, or cut the veil mix with more Glidden (...he will have 1/2 gallon left, ya know...) and then add a equal amount of the remaining Pearl & Poly as well. That will still maintain a easy roll-ability.
You see...there is a lot of flexibility in the suggested mix, and the OP has the ability to ask for advice at anytime. But he needs to get correct advice based on well documented results by others, and just because your seemingly new to realizing what the inherent properties of the suggested mix will have, that doesn't make then any less valid.