The long neck on the LNBF described are designed to get the front of the feed to the correct focal point.
There have been some feed designs that improve the impedance matching by use of a longer throat, often dealing with the removal of standing waves, filtering out of unwanted noise that might be a harmonic of the required frequency(ies), or there to improve linearity across the required spectrum.
However, unless you use a mechanical waveguide down to the pin diode at the bottom of the tube, and that waveguide is set to one polarity only, everything else is a compromise, and most likely the compromise is a profit in manufacture and distribution, rather than an improvement in reception.
