[mineption]

1. Encounters.
a. Are the encounters in general too easy? Too tough?
b. Are they easier at certain levels than others?

I'll get into detail on my opinion on each dungeon:

Bezantur Sewers
Recommended party members: 1-2

Bezantur Tombs
Recommended party members: 1-2

High Road Abandoned Mines
Recommended party members: 1-3

Thasselen Cemetary
Recommended party members: 1-2

Thasselen Citadel Dungeons
Recommended party members: 2-4

Murbant Sea Caves
Recommended party members: 2-3

Keep of Sorrows Warriors' Crypt
Recommended party members: 2-3

Nethjet Barrows/Catacombs
Recommended party members: 1-2

Nethwatch Keep Basement
Recommended party members: 4-5

c. For you with high-level PCs, are encounters a challenge at all anymore?
d. Does partying make encounters too easy? Should more or higher-level creature be spawned when in a party?

The challenge is still there but it definitely is easier. It all depends on where you go and how many party members you have. I put in a "recommended party members" number on each dungeon listed above, and if you brought more than that number to the dungeon it felt too easy. Too hard if you brought less. I think in the higher levels this number is more on the lower end (so bringing anyone along on a Bezantur Tombs run feels too easy). So there is a certain degree of challenge that is not scaling properly depending on the party and level.

I would welcome more challenging encounters when in a party, but I would hope that the rewards are scaled accordingly. Currently if you were to form a party of 5 similar-level companions, the challenge of encounters would go down significantly but the amount of loot you get is cut down by about 1/5 (as well as the chance of you finding the quest item you're after, assuming all members are looking for something). The exp and quest-item-finding bonus helps alleviate this. However, if the challenge of the encounters does not go down with a group, then some other benefit should be there to keep encouraging groups.

2. Creature selection.
a. Do you think Thay needs more creature varieties created?

Yes, for the same reasons you've stated.

b. If so, at what level(s) should these creatures be created?

Generally the high levels, but also at certain levels in certain dungeons to help alleviate the inconsistencies in desired difficulty.

3. Upper level PCs
Thoughts/comments?

Considering the amount of work ahead of you and how much you've done, I think it was the right decision. I'm just thankful that you gave us the heads up.

Edit by Thayan: Removed 'spoilers' about specific dungeon details to DM Forum.