You definetly dont need to lengthen it, there will be very little lost wort under the false bottom, typically less then 8oz.
It is actually far more likely that you will need to shorten the dip tube under the false bottom.
I think you issue might be the false bottom itself.
I had issues with my false bottom when I orginally got it.
I found the problem to be a clearance issue between the false bottom pickup tube and the bottom of the mash tun. It seemed that once the wet grain was placed on the false bottom it would flex just enough that the pickup tube was touching the bottom of the mash tun, basically sealing off the pickup tube and causing exceptionally slow draining. I remedied the problem by shortening the pickup tube under the false bottom. I cut it so that there was about 3/8"-1/2" of space between the pickup tube and the bottom of the mash tun. That small fix solved my problem and I have been doing full speed runoffs since.