With an endgame fleet and no solar shielding you can easily lose hundreds of supplies from a very short trip into the corona. I even had the industry skill that was supposed to reduce the effects of coronas (although I suspect it may have been bugged).
You're taking what I said out of context. I didn't explicitly describe my fleet, so I'll do so here: I had a Dominator, two Hammerheads, two Drovers, a Wolf, a Lasher and two Kites for combat ships The Hammerheads, Drovers and Dominator were all protected by Solar Shielding, plus I had the industry skill. That means that my CR loss for the protected ships would be y = x * .5 * .75. That means that the percentage of CR I was losing was a mere quarter of your percentage and only 12.5 percent of the standard. Obviously, in absolute terms, we're comparing apples to oranges, but in your case if the capital ships had been protected by solar shield, you might have lost fewer than one hundred supplies if you found yourself in a corona.
The issue here is that the ships you've said don't need shielding (d-hulls and civilian ships) are the ones that can afford to give up some OP since they are either expendable or they don't see combat. The high maintenance ships are the going to be the combat focused ships that need all the OP they have and more. 20-30 OP on a capital or cruiser is just not going to be available most of the time if you want a good combat focused load out.
Sure, if you spend 30 OP worth of Battlecruiser on solar shielding, you probably couldn't beat another BC one-on-one; Not without sustaining terrible damage - all other things being equal. But that's not really a realistic application is it? To be frank, I don't generally use solar shielding unless I am doing survey missions out on the fringe. I see it as a very specialized hull mod which is appropriate for that and not much else. And out on the fringe, there isn't much reason to bring an end-game fleet unless you want to waste a fortune in supplies and fuel. Specifically, the reason I don't use solar shielding on my eight or nine fuel tankers and three or four freighters is because they are hauling surveying equipment, which saves me boatloads more supplies than the shielding ever would, and insulated engines, because otherwise all those civvy hulls would be like running in the night, waving the Olympic Torch around, blowing beautiful sonatas on a vuvuzela.
And sure, occasionally you will locate a heavily fortified Remnant contingent, but in that case, you could stealthily survey the system and come back later with a strike force specifically outfitted for clearing that system; Much later when you aren't expecting to spend a whole cycle out there. For almost any other threat, a single cruiser, a pair of light carriers and some escorts - and really the escorts ending up carrying the majority of the engagements - can massacre just about anything else you could possibly run into out there and most of the time, you'll return home with more supplies than you started with.