As far as I recall - and it is some time back - I got through most of the USA sub missions first try. Can't recall this mission in particular, though I did manage first try when I did the above walkthrough a couple of days ago. And I figured that out in real time - I mostly don't like to follow other folk's guides
I do actually remember thinking the USA missions mostly seemed easier than the other two factions (but then, I had completed the other two factions first so I had a fair bit of sea-time under my belt).
I grant you, this is not an easy mission, but it is by no means extreme. Some of the surface missions, on the other hand, I would regard as bordering on the extreme!
I must stress that this is only my experience. Some people find some aspects of the game easier or harder than others. TAG did a trial at a local school once and there was one little girl who could take out a ship at max torpedo range every time - the AI programmer was most humbled!
Now I can't hit a barn door at 10 paces, so my main trouble is being so close my torpedos go under the target. However, I'm pretty good at knowing when to duck, dive, sneak, pop-up and let the buggers have it where they least expect it. I don't see more escorts as a threat, I see them as somewhere between an irritation and a bonus score
Practical advice? What I did when I got stuck on and frustrated by a mission was to play another faction, or surface instead of sub, or a patrol mission. After awhile I'd come back to the "impossible" mission and I'd usually crack it. The only mission I *really* struggled on was the MTB versus sub mission - I only got through that with excessive use of outside view and the pause key.
/bux