The Mac Pro will die. Move on. I do find a Quad core i7 (with 8 virtual cores for rendering and encoding) iMac readily handles pro level video work and I can't imagine it can't handle your audio needs on a power level. You can have gobs of RAM, plenty of harddrive expansion, and through thunderbolt, specialized add on cards. With neatly stacked external hardrives, it still takes up less space and you get an amazing 27" IPS display. Add on a second monitor just for fun.
Is the current high end iMac more powerful than your current 'Pro' system?
I suffered from the death of Final Cut Pro and moved to Adobe. However I didn't need to leave the platform.
Totally get what you are saying but actually for serious pro audio you need more RAM than the iMac can provide due to the use of RAM intensive sample libraries and also lots of Hard Drives - it will be interesting to see reports as more people try and build studios around Thunderbolt and what experience they have with daisy chaining and the bandwidth available for multiple drives and devices across 1 connection to an iMac - that is key. However this doesn't get over the RAM issue - you would have to create a slave computer with all your samples on and control it via ethernet which, while possible, ends up costing you more than 1 MAC Pro maxed out if you are clever about where you get your RAM from (ie not apple!!).