So after countless hours of research (you wouldn't believe how many I spent on mouse physical measurements and weight alone, as well as sensor and switch discussion), here's what I've found out.
MX300 - Can't get it new anymore. Very tiny, light, ambidextrous. No go.
G9x - Larger and heavier than MX300, right handed. Solid laser sensor. Expensive, but with a lot of features and buttons I don't need.
G500 - Currently using. Much larger and heavier than MX300, right handed, palm grip. Solid laser sensor, but wrong grip and size.
G400 - Larger and heavier than MX300, right handed, solid optical sensor. Not ideal grip.
CM Storm Spawn - Gaudy, slightly unsual shape, right handed. Good optical sensor. Larger than MX300.
Deathadder - Much larger and heavier than MX300, right handed, although good claw grip and great optical sensor. Too large compared to MX300 to work for me.
Kinzu v2 - Slightly larger and heavier than MX300, ambidextrous. OK sensor, lack of side buttons.
Sensei Raw - Larger and heavier than MX300, right handed. Good laser sensor. Bit expensive.
EC1 Evo - Larger and heavier than MX300, right handed. Good optical sensor, not OMRON switches, more tactile clicks.
EC2 Evo - Exactly the same as EC1 Evo except smaller, much closer to MX300 dimensions.
AM-GS - About as close to MX300 dimensions as EC2 Evo, but this one is ambidextrous.
Made my first narrowing of choices down to the AM-GS, EC2 Evo, Deathadder, Storm Spawn, and Kinzu.
Nixed the Kinzu due to the unremarkable performance and lack of side buttons. The Storm went next due to the unusual shape/size, and last I cut the Deathadder due to it being significantly larger than the MX300.
That left me with the AM-GS and EC2 Evo. Both mice are using the same hardware and are roughly the same size (closest to MX300 size of any of the above). The real big difference was in hand orientation, with the AM-GS being ambi and glossy, and the EC2 Evo being right handed. Well, I thought about it for a long time and while going with the AM-GS would make sense since it's ambi like my old MX300, honestly I'm right handed. I'm willing to bet that a right handed mouse would be more comfortable, and the difference in shape between the two was honestly very minimal.
So, I ordered the EC2 Evo on Amazon w/prime shipping. Hope to have it Tuesday/Wednesday, and I'll report a little review of the mouse after I spend some time with it.