If you run the CO2 tank vertically with a remote line in the hot weather, you should see about the same results as the HPA, given you are lucky with the expansion. The problem is that CO2 is unregulated, so it merely expands as needed. Liquid CO2 inside the marker may expand inside the marker, causing pressure spikes post vertical regulator. The vertical bottle with the remote line should not suck up liquid, only gas, and since it's warm out even if it did it would most likely expand before it even got to the bottomline. HPA will constantly be regulated (typically at 800-850psi) until the pressure in the bottle drops below the set output. Thus, your CO2 system on the same marker might put out closer to 1,000psi, 200psi more than the HPA. It could very well be a bigger difference, but I am only guessing from experience.
A force feed hopper will stop all chops, misfeeds, etc. Get a Reloader B, install a rip drive in case of jams, and you will have a hopper that you can keep and use on any marker you ever get. They're just plain good investments. I've even used them on pumps, on a low force setting, just to prevent hopper jams from the horribly inefficient gravity systems. You will see the most benefit on any higher end electro-pnuematic marker, but you will still see the benefits as soon as you realize you don't have to shake the gun as you're shooting.
Spring kit is always advised, coupled with a regulator. I would first adjust the velocity with the adjuster on the back of the marker to see if you can get it into acceptable range. Back it all the way out, then turn it in until proper velocity is achieved. I do not foresee you needing a spring kit, but it is a possibility if you just can't reach 285fps. I am not suggesting the spring kit or the regulator as much as I am the hopper, because like you said... you might as well just get another marker. A good regulator can always be taken to other guns but chances are anything you upgrade markers to will already have an equivalent regulator.