I just replaced my front speakers with Infinity Reference 6 1/2 inch coaxial shallow mount. I left the stock tweeters, as is, still connected, and my new speakers sound great. The efficiency must be pretty close to the stock as the volume is about the same....they sound brighter and clearer with more imaging.

The installation was pretty easy, tho I had to make my own template to drill the four lil holes....and then I used 1/2 inch nylon spacers with a separate rubber coated metal washer to make a spacer to raise the speaker 5/8 inch off of the sheet metal and stuffed up the extra space with packing foam. I used #6 1 inch screws. Total cost was $80 from Crutchfield plus shipping and maybe $5 for spacers, washers screws and new sharp drill bit (1/8 inch, I think ) They came with the proper wiring harness which snapped right in easily with no splicing.

Now to tackle the rears......