The Quad Core i7-3770S found in the iMac has a CPU score of 9098, a Xeon Six Core E5649 has a CPU score of 7037 and costs twice as much. However with the mixture of a 12 Core CPU, fast bus and PCI SSD there is no doubt the Mac Pro will be ridiculously fast. I however still prefer a multiple socket machine, for the price of one 12 Core Xeon I can buy two 6, maybe 8 Core CPU's with a rating of 10,000 and above and completely wipe the floor with it. My current HP Z800 has 2 Xeon X5660's with a CPU score of 8459 for each processor, the 12 Core Mac Pro Xeon will most likely have a CPU score between 12,000 - 14,000.
The top current Mac Pro uses X5675s, which are higher clocked than the X5660s that you have. The Passmark score incorrectly lists it lower. The new Mac Pro is expected to be 10% faster than this, which would be a score above 18,000. Two X5660s would be more expensive than the 12-core in the new Mac Pro. The E5-26xx line tops out at $2057. Your two X5660s are priced over $2400, though you may have paid less.
The E5-1650 would be good value but you can't use more than one. Two of all the other processors would be priced above $2057. There's no doubt that you can configure a two-core CPU to perform faster than a single-core but it's more expensive. Two of the Ivy Bridge equivalent of the E5-2687W would potentially 'wipe the floor' with the Mac Pro but the CPUs would be close to double the price as they are $1885 each.
It sounds like you are trying too hard to validate your own preference of buying a hacked together piece of junk as most people who do the same try to. Gamers do the same with consoles e.g why buy a playstation when you can knock together an awesome gaming PC for $500? If that's your preference, that's ok but you're kidding yourself if you think everyone else is making poorer choices.
I really like having a machine that can grow with my needs instead of chucking it out when I need something faster. Even if Apple allows us to upgrade the video cards or SSD, do to their design you will always have to buy expensive upgrades from them. Is the CPU soldered to the motherboard, I sure hope not but I wouldn't be surprised if it was.
You don't 'chuck it out', you sell it second-hand. Unlike the hacked-together abominations you'd be accustomed to dealing with, Macs have a thing called resale value. The problem with people who do things the way you do is that if you ever come to sell a machine, your target audience is people like you who know you can hack together your own computer anyway so the resale value tanks well below the lowest build cost of a new machine.
As for the upgradeability, Apple might solder things in place but these are machines with CPUs costing $2k. It would make more sense from their point of view to leave them so that the GPUs and CPUs can be reconfigured if there's a return. This could be done by switching out the boards of course but that still gives an opportunity for an upgrade as resellers can buy off-the-shelf Mac Pros and sell the separate individual parts.