Are upgraded power cables really worth the money?

Zeekzaak, it depends on your budget, but here are a couple that usually sell pretty well:
Shunyata (any of the models, but especially Python and Anaconda)
Purist Audio Dominus Ferrox or Fluid

These are good to try with money back guarantees (there are other excellent options as well):
DCCA
Tel Wire

I'm sure others will share some ideas.

Does anyone have experience with upgrading/altering the installation hardwire between the fusebox and the wall outlet? Apart from installing dedicated lines, I could consider higher quality of the wires themselves ...

In theory I geuss the power supply in the equipment can only see maybe 5 feet. If you have a upgraded power cord and or a conditioner that doesn't detract from the music, this is all you need, and dedicated lines. You can upgrade wire but its not really going to help. Use 10 gauge wire for you dedicated lines of 20 amps instead of 12. This will ensure less power loss accross the run.

beerdraft

I am amazed no one has mentioned upgrading your electrical circuitry as the biggest bang for buck improvement in this area. I spent a few hundred dollars and added 3 dedicated audio circuits to my main listening room. I think that is by far the most cost effective change you can make, and the one an audiophile should consider first. For about half the cost of a mid priced fancy cable, you can make a permanent change that will likely dwarf what any one cable can do for your system.
I have always considered Audiogon members a great source of information and inspiration - I guess this is a chance to "give back" to the community. BTW, all of these changes and improvements are so dependent on individual rooms, equipment, wiring, etc., so you still need to match the advice to your particular situation - there really are no absolutes.

Based on your components, I would try Audience powerChord. These should also do good on the amplifier. You should be able to determine with these power cords what they do and do you like what they do. they can easily be resold.