If I take a Mazda RX-7, swap a Toyota 3.0L V6 into it, and drop about 4-5k into the standard engine mods, I'll have a car with more power than a Mercedes SL600, that weighs 1500 pounds less, and has a better front/rear weight distribution. At the end of the day its still a mazda, but it'll still outperform anything you can throw at it. By saying "it's still a honda", you're just another one of those people who thinks brand name actually matters. If you want to judge based on the jokers who think that putting an AEM cold air intake and a "Greddy" decal on their civic makes their car nice, go ahead.
I'd love to see you spend that much money and get a car as luxurious, fast, and all around as enjoyable as a 120k mercedes. In fact, I'd put money on the fact that that is bullshit. I see enough cars and "project" cars everyday to know that there is only so much you can do to a car to make it better. If you go out and buy a honda civic and drop $50k into it, it is still a honda at the end of the day.
The only thing I'd say here about the Infinity Kappa series is becareful. I have hooked up a few systems that were all Infinity (my GFs car, for example, is all Infinity Kappa Perfect with Infinity amps and an Alpine head unit), and let me tell you those damn speakers are quite sensitive. If the proper air space and a certain level of air seal isn't present they can be a real bitch. That whole thing about rear speakers taking on low frequencies is some wierd thing that a lot of people think is true... I don't understand it. If you put the same speakers in your front and rear, they both do the same thing... and your rear speakers really don't matter at all anyway. More than 60% of what you hear comes from your front speakers and if you have subwoofers why do you need speakers to handle low ranges? The guys who win the nationwide sound quality comps every year have two speakers and a subwoofer in their car, that's it. Alpine is the shit, agreed. Also, your amps make all the difference in sound quality. They are just as important as the rest of the equipment in the system. Your whole system is only as good as the weakest link... if you bought $20 RCA interconnect cables, then your entire system is only as good as your $20 RCA interconnects. Rockford, btw, generally rates ther numbers a bit higher then they are capable of performing.
Regarding rear speakers:
They do get a lower portion of the sound. But the sound that they do get is heavily balanced towards the lower frequency range. If you have the same speakers front and rear, and you fade the music from the back towards the front, there is a noticable difference in the frequency balance you hear. The head unit I've tried this with was made by Alpine, so it's obviously a deliberate effect. I agree that rear speakers aren't necessary, but if I have some perfectly good speakers sitting around collecting dust then there's absolutely no harm in using them.
Obviously amps make a difference in sound quality, but when you look at the major respectable brands there's very little difference between, say, a Rockford Fosgate amp or an MTX amp. Go to any car audio board and ask them "what amp should I buy" and you'll either get 20 different answers or you'll get a lecture on how it really doesn't make that much of a difference. Obviously if you buy a $20 wal-mart amp it'll suck, but once you get to less than .1% THD it won't matter because the speakers will give you much more distortion than that. You're right about the weakest link of a system mattering, but a good amp is a stronger link than great speakers.
About Rockford amps, you're absolutely wrong. Every amp they sell comes with a "birth certificate" with the stats from when it was tested after it came out of the factory. The tested power is always higher than the product line's rated power. For example, I bought a Rockford 700S that was rated at 700 Watts, but when tested it produced 859. I asked around a lot and that's the norm rather than the exception.