Does anyone do speaker mods?

Speaker design is so complicated and so many factors are tuned in that changing a driver could make a mess out of everything, If you change the tweeter and it has more sensibility than the older one, higher frequencies will be louder and the speaker will shout at you, you would need to add some padding in the Xover changing resistor values, this requires a long time of listening and measuring and readjusting, you will probably need to buy 5 or 6 resistors to have the different values to change them around and cheap resistors sound horrible. A better tweeter will probably be more efficient.
Changing a woofer is worst because the cabinet is designed from the parameters of the woofer (small thiel factors) so you could get a better woofer and end up with worst sounding speakers.
Upgrading selected parts of the Xover is probably a good idea since caps could cost much more than the drivers, my guess is a serious loudspeaker company tunes different capacitors to the sound of their drivers, but I am sure some will use the cheapest Xover parts since we dont really get to see them.

Speaking as someone who builds speakers, I wouldn't say that removing the crossovers and sending them somewhere is very simple. Especially for those who have never opened up their speakers.

Jsadurni, I disagree that a better tweeter will be more efficient. It is completely driver dependent. Some are efficient, some are not. In addition to needing to potentially pad the tweeter (which is a good point) the crossover point could need to change. Which means complete crossover redesign on the tweeter side assuming it is a parallel crossover. If it is a series crossover, good luck. Some tweeters can cross low, some can not. You would have to know the optimal crossover point and find a driver that met those specifications. No simple task.

S7,

For the majority of speakers, we're talking about a few screws and a positive and negative wire to each speaker piece (tweeter, mid, woofer, etc.)

I don't think that's very difficult. You just have to make sure that you label the positive and negative wires and to which speaker piece they go to. Many xover PCB's have the labels on the PCB as well (to which speaker piece they go to.

Again, not very difficult. Pretty straight forward if you ask me. Heck, my 12yr and 14yr old sons understand it and have helped me from time to time.

I have a pair of Mirage M1's and I modified them by replacing the internal copper wiring with DH LABS Q10 Silver coated wire. The difference was amazing the pace tone improved dramaticly and the Bass was a big improvment. At first it was a little bright, but after it burned in, it smoothed out and sounded much more detailed!! It was a major improvment. Bob at audionut gave me a great deal on the cableing and lots of trusted advice on how to accomplish the task with out any problems. Id call him for any questions on speaker mods, or you can google him.

I've noticed modding speakers hurts their resale. For example people mod Klipsch speakers all the time but I notice when they sell them they seem to go for much less than comparable unmodded in equal condition. Something to think about.