So I replaced the original Garrard 401 mat with an EP

I have the transfi mat and it makes the sound thin and bright

Since 1977 I have been using the Hiraoka Diskmat SE-22 on my Ariston RD11S (cost new was $20!). Tonearms and cartridges have come and gone - yet this Japanese mat remains! I regard it as a perfectly neutral device, neither adding nor subtracting from the overall sound of the Ariston + arm and mc cartridge. I bought it new from Harvey Sound (48st NYC) in '77. Never felt the need to change or upgrade, unlike my other components! For me, it's a keeper!

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.

I replaced the original mat on my Garrard 301 with the SPEC+ Analog Disc Sheet (AP-UD1)

A nice improvement to the clarity and resolution.

Same... I replaced the original mat of my 301 with a SPEC+ mat and love the improvement.