[GroundUrMast]

Yes, if you are using a common 2-way splitter as a UHF/VHF signal combiner, replacing it with a UVSJ or similar product from another manufacturer will eliminate some needless loss and provide a net improvement to your system noise margin. You also reduce the interference between antennas. This is due to the filters in the UVSJ which are intended to block UHF signals entering the VHF port and VHF signals entering the UHF port. A single band antenna will receive some out of band energy and if allowed to mix with the primary signal from the correct antenna, can be enough interference to cause difficulty.

You're on the right track, trying to improve your system by eliminating unneeded losses, particularly near the antenna, is more effective than throwing active amplification at the problem. Very minute amounts of noise are generated by every atom in the universe. This is due to the fact that they all have moving electrons and each atom moves about even when it's a part of a larger mass. The result is that there is a low level of noise present, sort of like 'sea-level'. Even if there was no signal transmitted, an antenna will produce noise all by itself. That noise can never be eliminated, the desired signal must be stronger than the 'background noise' in order for successful reception to occur. Losses in cable and splitters will lower the level of the antenna noise power before it arrives at the tuner, but the cable and splitters also produce similar amounts of noise, so at the tuner, the background noise level can never drop below the minimum amount produced by the atoms of the cable or splitters. Meanwhile, if the desired signal has been attenuated enough, ratio between desired signal and noise may be too low for the tuner to use.

Again, trees and other vegetation present more trouble than a simple fixed loss. They produce a rapidly fluctuating amount of loss when moving in the wind, that can vary depending on the frequency. Adding passive gain (higher performance antenna, not amplifier gain) or removing loss may help, but there is not a guaranty that added fixed gain will overcome the variable component of the problem.