In your mild zone you can probably get away with chopping them down to a more uniform height, retrimming the sides, and then planting a few more to fill in the gaps. Red tips Photinia are really trees, so they're just doin' their thing! Keep in mind you will be doing annual pruning maintenance on these if you choose to keep them. They need at least a 6 to 8 ft wide area to fill in, otherwise they get leggy at the bottoms.

In your zone there are probably a million smaller screening evergreens that won't need as much attention.

I'd cut the photinia all the way to the ground now; they will grow about three or four feet just this year, but will be full and shrubby from the ground up. This is one method of rejuvenation pruning, and works extremely well with photinia.

It may also be important to note that photinias in th PNW are often afflicted with Entomosporium leaf spot, a particularly debilitating fungal disease that will typically defoliate the plants over time, weakening them to the point of seriously stunting growth or killing them. In many areas of the PNW, these shrubs are no longer recommended or sold for this reason.

You may want to reconsider your plant choice. We are not exactly limited to choices of broadleaved evergreen shrubs in this area and there are many that will present few of the problems that photinia does.

Yes: if your Fraser photinia hedge is not being thinned and spoiled by leaf spot now it will be later, based on how pervasive this has become over a large part of the US. Up my way (Seattle area) the plant has pretty much been rendered unusable.

Pacific wax myrtle hedges seem to have become favored, you might like to look into that one. It grows up pretty fast, has a perky appearance and appears to be very tolerant of shearing.