On the one or two occasions that I tried cherry-picking and/or game-saving my way to a supernatural tonnage score I found that it wasn't fun at all.
I think I managed once to top 100k without expressly attempting the feat, and it still felt kinda silly... at one point deck-gunning a massive passenger liner.
I did save a screen print for the tonnage, but never bothered to announce it anywhere cuz it wasn't something I was particularly proud of achieving.
To me, a more impressive feat would be to post pictures of manually targetted torpedo impacts on ridiculously small vessels at long range. I've toyed with the idea of creating a "Squirrel Hunter" mission where you have to sink an Elco with a torpedo. For some reason though I haven't gotten around to it...