The SWR will be higher in the phone portion of the band, and more than likely you will need a tuner to get full output from your transmitter. Generally the more L you add to the tuner the more loss you will have and consequently less power output. This is especially true with a tuner similar to the one used in the flex as it uses lumped capacitors and inductors. Generally the inductors will have a higher loss than a tuner comparable to the Palstar tuner that uses a large rotary inductor. If you can afford it, go with that kind of tuner. You probably won't get the same directivity operating in the phone bands because the relative tuning between the passive and active elements will be changed.
Norm - W7CK
So in the real world we use antenna tuners and low loss heliax to use our yagis at the ends of the band ranges and are generally satisfied with the results. Or use frequency agile yagis like the SteppIR antennas.