It took me a few minutes to figure out your math. Just to confirm, your calculation re: paying $372 extra is ...
$225 buy-up x 2 tickets - $78 already spent on E+?
If so, you should be aware that United officially considers E+ purchases to be non-refundable even in the case of an upgrade, so you may not be able to count on getting the $78 rebated. Unofficially, though, some people have had luck getting agents to refund E+ in the event of an upgrade.
Given that, based on your questions, you're non-elite, I think that the lowest fare bucket that will confirm you into first/business on that route is P, you should earn 150% RDM and EQM if you pay for an upgrade (but I'm not super-confident on that).
As for value, I think that this is something that you'll have to decide for yourself. If you're on a ticket in the E fare bucket, you've already paid quite a bit for your ticket, so it may be worth it to make the trip more special (especially if you're travelling with someone special) ... although, to be clear, first class on United isn't all that special anyway. Having said that, if you're on the fence and don't really mind sitting in E+, you may want to consider waiting until check-in to see if you get a buy-up offer. If you get an upgrade offer at check-in, I would expect it to be closer $100 per ticket ... just be sure to check in early so you're ahead of others who get similar offers.