2 things. the number of "ways" doesn't make the speaker better. in fact i generally stray away from the higher numbers as additional tweeters does not = better sound or even louder. The only benefit would be more efficiency as a 1 in tweeter would theoretically be less efficient at 20k than say a 1/2 in tweeter. also most companies know this fact so the 5 way or whatever speakers aren't generally any good to begin with since people who know its a marketing ploy don't buy into it

all that aside the main factor i would look at is if you are getting 4 ohm speakers or 2 ohm speakers. a vast majority of speakers are 4 ohm and if that's the route you are going than you get half the power the stock amp has to offer. while you likely will get a better speaker from the aftermarket speakers 1/2 the volume will either make that hard or impossible to tell (cracking up the power causes a dirty signal so a better speaker playing a dirty signal is not going to sound better than a worse speaker with a clean signal).

last but not least you can always consider an amp bypass to run the 4 ohm speakers off the internal amp in the radio directly either removing compeltely or rewiring to bypass the stock amp all together.

 

if you end up with 2 ohm speakers i would just replace the speakers see how it sounds then decide the amp portion later.

 

 

if you have a 100 dollar 2 way speaker and compare it with a 100 dollar 5 way speaker i would bet money the 2 way would sound better. If you can, go out and listen to a set installed at a local shop, best buy, walmart ect and judge for yourself.

 

EDIT: Got ninja'd by Emperor.
Depending on brand/materials, yes. I choose upgraded pioneer speakers myself, but wired them in place of the stock pioneer ones. If you want an even cleaner sound, component speakers are the way to go instead of the standard coaxial.

 

The best bang for buck performance on speakers maxes out on 3 way, after that your just looking at spending money on redundancy (multiple drivers that are in the same frequency range) and "cool design". You can get really good 2 way speakers for the same price as a 3 way but imo the highest quality stops at 3 way. With our cars having the tweeter by the mirror 2 way would be the better investment for the door, if you had a lt or ls where there wasnt a tweeter there then I would say go get a high quality 3 way. For 6x9 I say def go for a really nice 3 way since you have the real-estate there for good size drivers. Personally in my ls I have 2 way up front and 3 way in the back and it sounds really good. Almost as good as my stock ss, the only difference being I can tell im missing a little treble up front and bass from the rear since I dont have the tweeter and sub like the ss. I got fairly expensive kenwood performance series speakers in it and if i have to change out my ss stuff ill be going with kenwood again probably

 

 

The stock amp can handle a good amount of power. The only time you would upgrade the amp is if you start going into the higher voltage speakers/system, of which you'll also need a mono block for the upgraded sub.
My current setup, I'm using higher end Pioneer speakers both front and back, with a AVH-X2700BS head unit and the stock subwoofer. The only upgrade tier for me at this point is component, but when I do upgrade the sub by modifying the stock housing, I'll be wiring in a 300amp 2 channel mono block specifically for the sub. The stock amp still can power the speakers.