I have never seen RF performance degrade, but have seen connections stabilize with each new firmware. 90% of iOS updates come with firmware updates, so I fully expect it to happen, especially with 8.1.
Edit: iOS 8.1 betas have had new baseband firmware revisions. 1.02.02 in Beta 1 and 1.04.00 in beta 2.
For an excellent reference to iOS baseband firmware revisions in conjunction with iOS releases, check this site out: http://theiphonewiki.com/wiki/Firmware
For the Beta releases, see here: http://theiphonewiki.com/wiki/Beta_Firmware#iPhone_6
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