The trickiest part about IP redirection is that a lot of crawlers are US based, and therefore the crawlers may not see many of the pages that currently exist on your site and deserve crawling/ranking. There are cases that I've been involved in where IP detection was crucial, but I've also experienced problems with it.
The best advice I can give you is that if you must redirect, then monitor the website on a daily basis and if the site bounces up or down (all other factors equal), then look at removing the redirection.
Here's a thread that you might like to look over: http://productforums.google.com/forum/#!category-topic/webmasters/chit-chat/c2gk9cKWJYY
I really hope this helps!
