I believe the 5x7 stock speakers are 2 ohm. The ohm load is completely dependant upon the speaker. The amp operates at whatever impedence the load is. Let's say, if the amp is putting out 20 watts of continuous power with a 2 ohm speaker and you swapped that out for a 4 ohm speaker, the amp would only put out 12 or 13 watts.
This wattage difference is crucial because your perception of loudness is not on a linear scale, it's logarithmic. When you double power, you increase 3dB. That means that the difference between 1 and 2 watts is exactly the same amount of perceptible difference between 1000 and 2000 watts, it's an increase in 3dB. So, the difference between 12 or 13 watts and 20 watts is quite a lot. Also, at comfortable listening levels you're listening to a lot less power than you think, usually less than a watt.
High quality amps double their output when you halve the impedence, and that's something that is an indication of quality when looking at aftermarket amps. You want to see, as an example, 50 watts at 4 ohm, 100 watts at 2 ohm. Cheaper amps will say something like 50 watts at 4 ohm, 75 watts at 2 ohm. Well, I could rant about this stuff all day so I'll stop here.
2013 Race Red GT/CS 6MT; Elec Pkg; RAAMat on the doors; Morel Tempo 5x7 coaxials; GT500 axle-backs painted black