Maybe its my imagination but....
 

Is the game getting better at assigning more "appropriate" missions based on the capabilities of your sub?

I've messed around and created a few boats which you might end up seeing in the "playable Russian subs" mod, including basically the whole line of GUPPY conversions (minus the initial GUPPY Is as they had been brought up to II standard) and while doing the 1968 campaign with one of them, I noticed that I was almost exclusively getting assigned missions in and around the GIUK gap and Norwegian sea, like you would assume a diesel/electric sub on barrier patrol would.

This is a bit different from what I've seen before from this campaign where about every second mission sends you into the Barents or to stop an invasion force.

I have noticed this too. I like this, so you don't get missions like you mentioned, or TLAM strike missions for subs that dont have TLAMs...

welcome aboard!
 

It's been mentioned that the better your performance, the more aggressive missions you're assigned.

So theoretically a less capable submarine would accomplish less, and therefore be relegated to defensive actions more often.

Quite possible, I thought I had been doing well though but I did fail two missions (couldn't chase down a cruiser force and an Echo managed to get away after I fired Mk 37s from too long a distance). Still I'd like to consider other than that I've had a high success rate. Sunk a sub-tender, a lone Echo, then another Echo escorted by two Novembers, a Spetznaz insertion team with a Foxtrot and a Romeo, and then another mission where I sunk a Juliet and evaded a November.

Not too shabby for a GUPPY IIA if I do say so myself. Sadly the skids came off the next time I was assigned a tanker mission. A Ka-25 found me and I couldn't get up to speed (which is about 14 knots in a GUPPY) in time to try to evade, so that was that.

Mission successes aside, performance could be related to tonnage sunk as well, but don't quote me on that.