Using Filtfilt function for OFDM signals

Hello:
I have a question regarding filtfilt function in Matlab. Due to forward
and backward filtering, filtfilt function will have zero-phase response at
any frequency. If I apply this filter to a time domain OFDM sequence, does
that mean I can set the FFT start window the same as the transmitted IFFT
starting point?

In my test, I have an OFDM sequence of 5 symbols each with 80 samples at
20 MHz clock. Only the third symbol has valid data, all others set to zero.
After passing though a 5th order Butterworth filter implemented  using
filtfilt function, the data are spread over previous symbols and following
symbols. But for filtfilt, there should be no phase distortion. What the
data outside of 3th symbol represent? Should I call them intersymbol
interference? Does the third symbol itself suffer from intercarrier
interference as well?

I have another question about phase delay and group delay.  We know linear
phase FIR filter has a constant group delay. If the phase response is
(angle(w)=a*w). The phase delay and group delay both equals a. There is no
time delay distortion or envelop delay distortion. If (angle(w)=a*w+c). The
group delay is still a, and it is a linear phase filter. But the phase
delay (-angle(w)/w) is not a constant among frequencies. Does this filter
has phase distortions? 

Thanks very much

X.Y