Well, after considering all of the responses to my post, I pulled the trigger on a pair of Infinity 6912M 6X9's to replace my factory Clarion 6.5's.
Wow! What a huge difference (not surprisingly) they made. I can now actually hear my music while underway. While I have no way to know how they sound vs. the Polk 6.5's that are so popular on this board, there is no comparison to my old speakers.
Installation took me about 20 minutes for both sides because the blade on my saber saw could have been sharper, but everything generally went rather smoothly.
I did cover the top of the speaker area with 3/4 plywood which I secured using acrylic latex silicone calk, so each speaker now has a "speaker box", and I have some additional storage area on top of the speakers as a consequence.
The speakers sound quite good in my opinion and do have a fair amount of (mid?)bass, although obviously not when compared to a subwoofer. All in all, I think I'm glad I went with the 6X9's.

Before

Enlarging the hole

Finished!

Enjoying the jams...