Well there are a zillion different strings out there. I don't mean to open up a can of worms, especially if you really like the NXT, but one solution to the tradeoff of tension loss versus restringing cost is to find a good, durable polyester string you like. Some of them can be pretty resistant to tension loss, but only time and experimentation will let you know for sure. The same string at the same tension will feel completely different to different players.
Three months is a pretty long time to go between re-strings, but as long as you're not getting a noticeable tension drop or seeing a lot of fraying, that's OK. It just means you need to play more tennis.
For me, the key indicator is when I have to start sliding my dampener back down or adjusting my strings every several points.
I think a lot of players tend to string a little higher than what they like as an optimum just as you said--thinking that after a couple of hours it will drop down in tension and be perfect for awhile, before they need to go for a re-string, and save a little $. I have heard different theories on this--my stringer actually pre-stretches the gut and advised me that if it were strung higher, it would feel a lot different coming down to 57 than pre-stretched and initially strung at 57. Same for the poly when it's initially strung at 53, as opposed to "dropping down" to 53. It just plays different.