Are there any 2-ohm speakers on the market to do a direct replacement for the stock ones in the G?

 

Image Dynamics

http://miata.cardomain.com/item/IDSCXS62

 

I've been looking at those. They seem to get pretty good reviews. Not sure if by "direct replacement" you meant that you'd keep the stock bose amp and just change the speakers, but those Image Dynamics have a RMS rating of 150 watts. You'd definitely need an aftermarket amp.

 

I wonder if there are any amps that can fit in the same slot as the stock one. I don't have to have to do any drilling or extra mounting.

 

Ummm...no. The only way I'd recommend those speakers is if you are planning on upgrading the amp eventually. They are a good base to start with. You will not find a amp that will "replace" the factory amp...you will have to do a little more than that. Image Dynamics makes GREAT speakers.