The single best improvement you can do are to use a low noise amplifier.
You can use the RG6 coax and for the money saved from not going LMR400 you can buy an amplifier and a splitter.
Instead of speculations I can provide you with the facts. Below you will see the impedance that the SDS100 will have from 700 to 900MHz. It swings between 30 ohm and 120 ohm. A discone like the Sirio SD2000 have a SWR that swings between 1.0 and 3.5 in the same frequency band.
That 3.5 SWR equals to a 110 ohm mismatch to a 50 ohm coax.
When using an amplifier at the antenna it will work as an impedance buffer so that the coax will see a constant 50 ohm or 75 ohm impedance depending of if you use an amplifier designed for 50 or 75 ohm.
A CATV splitter at the scanner will also work as a buffer and makes the coax see an almost constant impedance of 75 ohm, instead of the scanners 30-120 ohm. There are also 50 ohm splitters to be had but are often more expensive. The middle of the scanners 30-120 ohm are actually 75 ohm.
The additional benefit are that you also do not have to worry about any coax loss and if the amplifier are low noise it will also improve your scanners reception as all scanners have at least a 3-4dB noise figure, usually more than that, and low noise amps have a 0,5 to 1dB noise figure. The total system noise figure will be the amplifiers plus something like 1/10th of a dB. Just from the noise figure alone you get 3-4 dB increase in signal sensitivity. That's like using twice as big antenna or stacking together two antennas. Then add the coax loss that you do not have any more and the improvement will be even bigger.
You will have to reduce the signal level to the scanner or it will overload. Either choose a splitter with a high attenuation like 10-15dB or use an additional attenuator, preferable a variable one that can be set more precisly.
Using any high quality/cost coax will always have a loss and the impedance mismatch between scanner-coax-antenna will still be there if you do not use a splitter and amplifier.
/Ubbe