Lots of guys in europe use W- and T-series combines in corn without any problems at all.
Here is a short video of the 5-walker machine T560 hooked up to an 8 row kemper header. http://www.youtube.com/watch?v=Pe6rG6JgPRM
The T670 works fine threshing corn, but it is VERY noisy in the cab. I was riding with a neighbor one day and you could not here the radio when it was turned up all the way. The cobs hit what sounds like the bottom of the cab. This machine was not built for corn.
batcher13 did your neighbor's combine have a variable speed feederhouse? Because according to Deere's ad literature the t670 can only be ordered with a fixed speed feederhouse. Just wondering how you can combine corn without a variable speed feederhouse.