That mod doesn't even feel like Grimrock, but it's pretty amazing. For a tougher Grimrock challenge, there's the "Master Quest" by Komag that takes the original game and amps it up (the enemies even scale with your party's level); sounds like you'd be into it:

"Legend of Grimrock - Master Quest is a complete re-working of the entire original game with tons of new puzzles, updated dungeon designs, unique story elements, and more. It has been referred to as "Grimrock on Steroids".
- The overarching dungeon layout is similar, but there are lots of local changes to specific rooms, and some completely new areas have been added as well.
- Original assets were used for the most part, with a few notable exceptions (such as Daniv's excellent wall set variations).
- Most major items have been moved.
- Puzzles have been generally "upgraded".
- The base difficulty level is increased:
* stronger monsters
* more monsters
* trickier puzzles (some with added clues)
- The difficulty (and XP awarded) scales up dramatically according to the average level of the champions at the start, so you can import your original Grimrock dungeon party and continue your adventure and advancement. "

Steam - http://steamcommunity.com/sharedfiles/filedetails/?id=132493547
Nexus - http://grimrock.nexusmods.com/mods/239/?

Yep, that was the one I was looking at, actually. Sounds fantastic :)

But I'll definitely check the one you suggested. "That mod doesn't even feel like Grimrock..." is actually a positive. The game is great, but the replay value is diminished by the fact that you already know what's coming and there are no surprises left. A mod with a different feel, but using the cool mechanics of LoG is a win in my book!

Then wait until you actually play it...I know how much of it was done, and I still can't believe it. Feels completely different.

As for Komag's riff on the original game, even he has differences, including some open sky areas. I'm curious as to what you think (and what you play)...there's some incredible mods out there.