So I replaced the original Garrard 401 mat with an EP

Hey Uberwaltz. Since I replaced the platter with the PAC 20mm oversized aluminium (it's UK made!) one, I prefer no mat. Rings like a bell -by design- until the record is placed on it. Sounds by far the most resolving.

Did some back and forth and can confirm the result of removing the mat and replacing it with a 10" record:
1. Warmth - a slightly yellow-brown 'coloration' has been removed
2. Heaviness - with mat, it sounds like the musicians are behind a curtain and on a smaller stage
3. Brighter - As @pani said, it is definitely brighter - but much clearer. Rim of drum and plucked bass distinct, clear and fixed in space.
4. More air, depth, and dark space between instruments instead of a wall of warm sound - less lush. plucked strings snap into space in a startling manner. 
5. Increased pace - I had to test speed with strobe as I was sure it was running faster without heavier mat - it was exact.
6. Sounds louder - confirmed by others present.
Notes:
a. I can understand bright recordings being irritating but the improvement is too much to ignore.
b. I am convinced the mat damps and veils the sound. It's like going from Mullard to Telefunken.
c. Surface noise frequency spectrum shifted up slightly.

Thanks for the recommendation. Reading about the Spec+ gave me an idea.
I just swapped the 10" vinyl record (Daughter - His Young Heart EP) for a 10" shellac 78.
Wow! All the improvements listed above even more pronounced:
1. Lower surface noise
2. Improved depth
3. Hearing further into recording
4. Better focus and more neutral.

@normansizemore 
Yeah, I thought the general consensus was leave the original mat. But I have to admit the improvement is not small. I've swapped between the original, the 10" vinyl and the 78 using a number of tracks. Considering I've a stash of old shellac 78s, I think I'll repackage them as a miracle mat for $300!!

I meant to follow up earlier. I've been swapping between all 3 and have gone back to the original mat for the moment! The oddest part to all this is that sonic improvements depend on the record/recording. The changes to the sound depending on which mat is used are ridiculous - making most records sound utterly different. Depth, timbre, space, tone, clarity, blackness, silence all vary. In fact, so much so, I wonder which one is correct!

Forgot about the thread. I did some more tests and am now staying with the 10" EP as the best compromise.