From a systems standpoint, if your copper cactus is allowing you to monitor all that you need to hear, then there is no need for higher performance.
One thing to remember is that when you lengthen an antenna beyond a quarter-wave vertical, or a half-wave dipole, the directional characteristics change, among other things. This may or may not be a problem depending on your needs, especially for rx-only.
Example: Your copper cactus half-wave has a directional pattern at VHF that looks out mostly horizontally towards the horizon, and not much sensitivity to signals arriving from straight above. Yet I'll bet that you can even hear UHF comms 446 mhz ok too. However, if you were able to actually "see" the directional pattern at 446 mhz in front of you with your 2-meter cactus, you'd see a "cloverleaf" pattern more or less so the antenna will respond best to UHF signals arriving from about 45 degrees above the horizon. This might be ok for UHF repeaters if you are in a valley, but the mobiles will be weaker than had you used a half-wave cactus cut specifically for uhf.
Some are actually able to accept this higher look angle on uhf and put their standard 2-meter j-pole into dual-band use. Not ideal, but it works for some.
Now on 800mhz, even if you have a good match on with your existing cactus, and low loss coax, your antenna is looking for the strongest reception practically straight up and not at the horizon. So the 800mhz systems you are hearing are just really strong. If you had an antenna cut specifically for 800mhz, say a simple 6-inch vertical dipole, the strongest reception would be at the horizon again, and not straight up, like your cactus is doing now.
This is similar to the popular "OCFD" vertical homebrew wideband dipole here which is about 4-1/2 feet long. Yes, it matches well across a wide spectrum, BUT as you go higher in frequency beyond VHF, the antenna is looking high in the sky for best reception. That compromise is ok for some, but most would not claim it to be "high performance" on it's own merits without taking into account the "system" that works for you.
One thing that might be helping you, is that at 800 mhz, the common-mode of the coax will be skewing the directional pattern of your 2-meter cactus for a lower look angle! I haven't seen any common-mode chokes that work at 800mhz, so your transmission line might actually be assisting in your 800 mhz reception by unintentionally skewing the pattern more favorably. Usually this isn't desired. The "OCFD" suffers from this same issue, but if the unintenional skewing helps, then it's all good.
To sum up - when the antenna is longer than normal, the best reception is usually no longer broadside to the element, but starts to move more towards the ends. Whether this is an aid or a hindrance is part of the design decision.