Did you find any cause for the iPad mini poor roaming? I have a similar issue and also a problem with the ipad mini disconnecting when other devices are maintaining solid connection ( iPad 1 and iPhones a ok)
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You need to provide more information as we don't know where the problem is.
The decision to which AP to join has got nothing to do with the AP or your network. The decision to join which AP relies solely on the wireless client itself.
For Apple products, either use only of the the two: WPA or WPA2. Do not enable both and hope that the wireless client will choose because Apple products DON'T like to choose.
Look at this table http://support.apple.com/kb/HT5535 check the Ipad Mini software and compare with the other's.