Express A1392 vs. Extreme a1408

Strictly speaking I'd say no. A new Extreme is likely to result in somewhat better coverage, but it won't be a quantum leap.

Since the nature of wireless communications is such that it's impossible to determine in advance how well it will work in your circumstances, the only way to be certain is to try it. You can always return it within Apple's return policy if you determine the expense is not justified.

My suggestion is to leave your existing equipment in place, but add another cable to another Extreme (or Express) connected either to the one upstairs or in the basement, whichever one is more convenient for you. That is certain to help.

If by "extender" you mean you are using it to "wirelessly extend" your Apple AirPort - created wireless network, that's always a compromise with limited benefit. In that case a newer Extreme is unlikely to provide any additional benefit.

The Extreme may provide somewhat better wireless coverage due to improvements in its antenna design. My own testing indicates the difference is quantifiable but not significant.

Make sure the AirPort Express is configured correctly. If powering it down results in a reduction in your overall wireless service area, then it is configured correctly. If powering it down results in nothing you can observe, then it requires further investigation.

Sure, that makes sense, but why not simply add another Express or Extreme to your network? That will undoubtedly improve your wireless service area. You can keep your present equipment in place, just add to it.

A basement is typically the worst place for a wireless access point, unless it's a finished basement with equipment requiring a wireless network, but you already know that. An ISP's installer, being lazy and obligated to do nothing more, will generally bring about three feet of cable into a dwelling. The service entrance location is almost always in a basement or remote corner of a house, and wireless access points should be installed in locations central to the equipment that uses it. Buying additional cable to bring it there is cheap.

Was wondering if I should replace the express with an a1408 or 6th gen extreme to improve performance. Does that make sense

Honestly, I cannot detect any signficant difference between an AirPort Express and AirPort Extreme as far as wireless range is concerned.....and I'm talking about the "new" AirPort Extreme here. I did not do exact tests, but would estimate....in my home....that the Extreme was maybe 5-10% better, range wise.

If you are saying that the AirPort Express on the main floor does not give you the coverage that you need on an upper floor (assuming you have one), or coverage in another part of the home on the main floor, then you are going to need to add another Express or Extreme in the area that needs more coverage.

If you have Ethernet devices, the Extreme is something to consider since it has much faster Ethernet ports than the Express, which is knocking everything down to 100 Mbps when the signal must pass through its Ethernet port. The ports on the Extreme will handle up to 1,000 Mbps connections.