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It wouldn't be "bad" at all. It will be different in that at 2911 has much more performance (about 3x) than a 2811 (i.e. you should see a lower CPU load for the same traffic levels) and you may see IOS feature difference if the two routers run different IOS versions (although using the same common features they should [but not always] behave alike).
PS:
Since newer router models are usually "faster", if you've found your 2811's performance acceptable, and you don't forsee bandwidth upgrades, you might also consider a newer router with less performance than a 2911 but still faster than a 2811, for example the 2901, 1941 or 1921.
