I never give any "bump up" for a certain coin. However, if a F sells for 5 times what a VG sells for, I take a hard look at it and if in doubt, (like this coin could be), I go lower. Yes, the grade is the grade regardless of the date, but if there is a huge price spread, I am more critical of a coin and make sure its everything for a grade it should be to command the premium. If I am wrong on this coin, and its really a VG, who really cares? A buyer of a 27s though would care. I guess you could say I am the opposite of what TPG companies are accused of doing. P.S. Btw I agree with you asking why someone would give a SL quarter a bump up because its "old". "Old", lol, I am just as harsh on 2500 year old coins as I am on any modern coins, (and to me 'modern" means anything struck with a machine).
I guess, IMO, the grade is the grade and shouldn't matter what the price is or the difference of price between your grade and the one higher or lower.
I think what you really mean is that you'd be more critical of a coin where the spread means a lot of money and to this I agree. There is much more at stake. But the grade is the grade regardless of the value and if you want to be more generous to a coin because it doesn't make a difference in value then I say that you're just confusing the person who buys it. That buyer is going to expect consistent grading when he goes to acquire other coins in the series especially the 27-S.
What I mean is I go through, look at it, and grade it. However, if there is a large gap, I would look at it again and possibly err on the side of conservatism if there was a doubt. The OP's coin, I would go through and grade a F12 not paying attention to anything but the coin. After, if I noticed the date and saw there was tremendous grade rarity between the two grades, I would take another look to ensure I would be happy guaranteeing to another collector this coin was indeed a F and worthy of the premium. I am just saying I would give it another look due to the high grade rarity implied by the pricing, just to make sure my original analysis was correct, and the coin is graded correctly. If this coin was a 27s, I might pull out some other auction listings of examples, look up certified examples to see what they look like, etc. Maybe its wrong, but I just take extra care on items with such grade rarity to ensure I am correct, and don't assume my first grade has to stand.