Well, I thought the exact same thing in buying a maxed out iMac to run Avid. It will work fine if you use the iMac's internal sound card, but if you are planning to use an external audio interface connected to studio monitors, then it will have problems when you connect an inteface via the Firewire 800 port on the back of the iMac. Bottom line, Avid MC7 does not work well with Firewire 800! I ran an FW400 to FW800 cable from my fApogee Ensemble which is connected to Mackie monitors and Avid began making a really bad motor boating sound when I routed the audio signal to the Apogee interface. I spent nearly a month working with Avid until we finally narrowed down the issue to the FW800 port on the back of the iMac.
The best solution for this, especially if you want to import video and audio via the same interface, is to purchase the much anticipated Motu HDX-SDI Thunderbolt interface which is about to be released in next month or Oct at the latest. This is the first Thunderbolt interface by Motu and apparently they collaborated with Avid on the design. Plus, the price point is way lower than Avid video interfaces. Rather than paying Avid's very high cost for their video interfaces $4000 to $6000, it looks like the HDX-SDI Thunderbolt interface will come in at $999 retail, which means you can probably pick one up for $799 street. The beauty of Moto the gear is the combination of both great audio and video on the same interface. Mark of the Unicorn has been around forever and is known for quality workhorse audio interfaces, plus they put a cool time code display on the front and even some legacy connectors for old cameras too, S-Video, Composite so you can upconvert your old footage. Check it out man!
Here's a link http://www.motu.com/video-products/hdx-sdi-thunderbolt