Eclipse makes a amazing pointsource 6x9" speaker. This is a true dedicated component speaker with crossover. www.eclipse-web.com for more info, model# is SP6900.

Morel and Focal may also have a top notch set of speakers for you.

 

If your car takes a 6x8, then get a 6x8. There is little point in going through the effort to shovel in a 6x9.

I suggest that if you have RF front end, then look for an RF 6x8.

 

Dukk has a point...

I find if it is a Ford, then 6x8 works really well for mid and treble; however, a big advantage of having a 6x9 is the fact it is much more efficient (ie if the 6x8 is 90 db, the 6x9 is 92-93db). Also in a sedan 6x9 speakers seem to add a nice "punch" to the soundfield -- in fact I would not have treble coming from them in a coupe or sedan unless it was for a 5.1 system or your misguided preference.