Scanner antennas don't have to be of a particular 'size' to work. Being made/cut for a specific band is certainly nice, but not absolutely necessary for best results. And sometimes, having a separate antenna for each band of interest get's a little impractical. So, a 'multiband' scanner antenna is a fairly nice idea.
A 'disk-cone' type antenna does very well (I don't care who makes it). Because of the elements size (and shape) relationship they result in a compatible impedance match for several bands. While most receivers are more than sensitive enough, eliminating a little signal loss from an impedance mismatch is worth considering. They are not 'miracle' workers by any means though.
The higher you go in frequency the more important height becomes rather than just the gain of an antenna. At 800 Mhz, that "height makes might" is really true. 'Line of sight' sort of stuff. If both the transmitting and receiving antennas are above the 'clutter', things typically do very well until the curvature of the earth and/or something get's in the way. It depends on the terrain, of course, but I'd think that an antenna at 45 feet or so ought'a do pretty good! (I know I wish I had something that high.)
If 800 Mhz is the range you are particularly interested in, another thought would be a vertical array. A 1/2 wave length is something in the neighborhood of 7", so an antenna such as that wouldn't be all that 'obtrusive', as in sort of 'smallish'. A 5 element vertical array would be just over a yard tall, right? No idea what sort of gain that would give you, but it would definitely beat a disk-cone antenna.
Another option would be a remote pre-amplifier for 800 Mhz at the antenna. The cable companies do that sort of thing around here. Lots of 'catches' with that sort of thingy, but still an option.
Your LMR-400 is really about as 'practical' as a larger $ize feed line, and I'm not too $ure I'd even use that, afford that, would be a better way of putting it. (But then, I'm not all that interested in 800 Mhz, it's not used much around here.)
You have lots of options for 'improving' your system. Some are more practical than others, and that's something you have to decide, that definition of 'practical'.
Good luck and have fun.
- 'Doc