Yes, they would still work well, even on an external amp. They are just lower impedence than most aftermarket car speakers. If an amp is hooked up to a 2ohm speaker, it puts out a lot more power than when connected to a 4ohm speaker. The amp runs cooler with a higher impedence speaker, but puts out less power.
However, if you're going to run them from an external amp, the 2ohm aspect of the two speakers I listed isn't as important. They're good speakers but you can get away with running better speakers if you're going to use an amp. Look at Focal, Image Dynamics, Morel, MB Quart, JL, Boston SR series, and so forth. These are speakers that you wouldn't want to run off the head unit. How much do you want to spend on speakers (the ultimate question). If it's going to be a long time before you get an amp, I would still stick to the two I listed, even with an aftermarket head unit. Also, Polk makes a 5x7 that's 2.7 ohm that folks seem to like even more than the Infinity.
2013 Race Red GT/CS 6MT; Elec Pkg; RAAMat on the doors; Morel Tempo 5x7 coaxials; GT500 axle-backs painted black