Looking at that Panorama BS-800 it definitely looks like a great antenna, but then I think about that price and realize - and this is my own opinion so take it as such - I'd never pay that kind of price for such an antenna knowing that I could build one myself (literally) in 20 minutes that would probably work just as well if not even better: a 1/4 wave ground plane, made from an SO-239 chassis mount that costs like $4 and some #12-14-16 gauge electrical wire or even some cut sections of coat hangers.
And I'm not kidding in the slightest.
The issue I think you're having it the distance you mentioned which is quite a chunk to be receiving 700/800 MHz communications since those frequencies don't really propagate all that well as such distances and you could also have obstructions in the signal path from the transmitter sites which are going to reduce the signal strength even more. From what I've gathered in research about the Scantenna over the decades - even in spite of it's popularity - is that it does have some weaknesses such as it not truly being a very omnidirectional antenna, especially in the higher frequencies you're hoping to monitor (because they are highly directional as it is).
The Panorama antenna is nothing special when you look at the design, and to be perfectly honest if you could locate an old cellular antenna (mag mount, clip on, whatever) you'd do pretty well in the 800 MHz range but not quite so well in the 700 MHz (since it would be cut for about 880 MHz).
The idea of making your own 1/4 wave ground plane gives you a) something to do, b) something to be proud of, c) something you can experiment with and build more because they
are so cost effective and pretty fun to make, and d) cut it to precisely the frequency you want. In fact, you could make one cut to 772 MHz and also one cut to 855 MHz and use both with a coupler and probably do just fine. If you just make one antenna you'd want to cut it for about 820 MHz so it would be effective for both the 770 MHz and 850 MHz bands pretty equally.
But that distance is always going to be a major issue and if the transmitters sites and systems you wish to monitor really are 30+ miles away you have two options: get a signal amp + either buy or build a 1/4 wave ground plane antenna cut to 820 MHz to get as much of the signal to the scanner as you can or (maybe) get a signal amplifier + a Yagi antenna you can either buy or build (yes you can build those too) cut to the same 820 MHz center frequency and point it as required to really pull in the best signal you can.
A 1/2 wave ground plane might offer some better performance but I can't speak for the exact amount or offer specific figures in terms of decibels for signal strength, obviously. The distance from the transmitter site (the biggest concern), the antenna (next biggest), the height you place it at (a major factor), the coax you use as the feedline (pretty important to offset potential loss because of the signal propagation distance but LMR-400 should be excellent for that purpose as long as you're careful about the final length), it all works together to either provide you something the scanner can make use of or hearing more static than voice but I'm pretty sure you already know all this.
I'd say build you own, in fact build a few different ones on the cheap, see what happens, if nothing you create is functional to improve things, then consider a commercial antenna (hopefully one that's not quite that expensive) and of course a high quality LNA someplace in the signal path could offer some improvement as well.