Absolutely right. My view, based on film as a "sensor" is that the camera body is a way of holding the shutter open for the right length of time, and a support system of the lens. I might be exaggerating a bit, but not much - if the lens is good and you expose for the right length of time with the right aperture, you get a good photo (well, you get a correct one .... whether it's good or not depends on the photographer's skill.
But these days, it's more complicated, due to the nature of the sensor, noise from background electronics, the amount and quality of in-camera processing, etc. But at the end of the day, a £300 body with a high quality lens is pretty certain (if used correctly) to produce a better quality image (sharper, less CA, less vignetting, etc) than a £5000 body if the less is bottletop quality.
Of course, you can get third-party lenses that are VERY good optically, and compare well with Canon/Nikon pro lenses at a fraction of the price, even if they aren't built as well.
Again, agreed. Up to a point. Yes, certainly, the more you spend on a system, the better the chance of getting improvement on decent cables, etc. The marginal benefit logic still holds, though, and not just for the cables, but for the amp/CD, etc. If you (or me anyway, and I presume you) listen to a good £100 CD player and compare it to a good £300 CD player, you'll hear a marked difference. But that £200 buys you little or nothing if you're starting with a £2000 player, and comparing it to a £2200 player. You have to spend a LOT more to get a proportionate improvement ..... and in fact, by that time, I'm not sure you can get as much of an improvement as you could by throwing £200-£300 more at a £100 player. All this assumes, of course, that amp, speakers and most critically, the listener's ears, are capable of not being the limiting factor. There's little point in replacing a £1000 player with a £2500 player if it's running through a £100 amp and a £50 pair of speakers. Obviously, a system needs balance.
Yeah, probably. My view is that at that level, it's as much about what sort of sound you like as it is whether cable A is better than B or not. For instance, I like bass to be deep, but not boomy. I find a lot (but not all) modern speakers to be boomy, and far too many amps don't keep the bass as tight as I like.
I know what I like, and I know how to find it. I use a couple of Bach organ pieces to test bass, and I use a particular soprano (Emma Kirkby, on Bach's Jauchzet Gott in Allen Landen) to test tweeter purity and cleanness. Ironically, though, despite how that sounds, most of my listening isn't classical.
My point, though, was that I know what I like, in terms of sound, but it doesn't mean you would like the same, or that it's right or wrong. It's just what I like. And my choice of whether a speaker cable (or interconnect, or amp, CD, etc) is worth the money is based on whether it gives me money for value. For instance, my CD player budget tops out at about £500 (with a rather venerable old Marantz KI-Signature deck), but I spent more than that on the cartridge for a Mitchell turntable.
PC stuff is a bit less subjective, though. Whether a hard drive's capacity per extra pound is worth it or not is subjective, but how much capacity (and performance) you get is pretty measurable. Broadly, I feel the same is true for processors. That's largely why I'm not enamoured of the price-performance from i7. We can certainly benchmark performance gains, but in real world use, how much benefit do we get? And that's why I say, for me, i7 isn't the financially smart move right now, because I don't see the value for money. But you, or others, might feel you do get value for money, just like you might not like my choice of speaker - your value judgements are different. Neither view is right nor wrong - it's down to what value we each place on things like upgradeability. My perspective is that future proofing is often, or usually, largely illusory, so I don't put much of a value on it. I buy for what will do the job I need doing now, and for my projected lifespan of a machine, which will be several years. And I don't see what I will need an i7 for that a good 775 won't do, now and for several years, and for a fair bit less money. Which leaves me scope for a new lens.