I don't think you stand to gain much from your efforts. Two reasons come to mind.
First off, powder coating is pretty much just finely ground plastic. It adheres by static and is then melted to create a plastic coating around the part. As a general rule paint does not adhere well to plastic. You can get a shellac based primer to adhere, but shellac base primers don't tend to be good for use in exterior applications, and it sounds like your exterior application is the sort of thing a paint manufacturer would call an "extreme environment". You have the deck stacked against you on that front.
Second issue is probably even bigger. You need proper prep and priming underneath that powder coating before the exterior surface is even worth worrying about. Odds are good the manufacturer has done nothing more than a chemical dip as a cleaning and has done no primer at all. Proper prep starts with breaking all hard corners. At most a manufacturer will de-burr the edges and tumble the parts to do this, but I doubt that is going to be done for a powder coated part. Your powder coating failure will usually start at a corner. Paint will not adhere well to such surfaces. It draws back from them and only the thinnest amount will be present (it is a surface tension issue...I have forgotten the technical term for it, but powder coating is susceptible to it as well as is solvent base). Rust gets started in such a spot and then creeps its way along underneath the coating. This is why a good primer is an essential element of the process.
If you truly want a durable finish your best bet is to take your new parts and go after all edges with a file. Tubular parts are easy. All you have to worry about are the the ends. That probably means whatever brackets have been welded on or whatever inserts have been welded in. Round over all the edges. You don't want anything with a ninety degree corner or point on it.
Once you have softened all those corners then sandblast the parts to get back to bare metal. That will probably be a pain to do. No one likes to remove powder coating that is still in good condition, but it needs to be done so that you can get a good zinc-base primer onto the metal. Only after you have a good primer down should you start fretting over top coats. (A rust converter type primer might be a good alternative to a zinc based primer, but I am not up to speed on those new-fangled products--do some research.)
Personally, I would not bother to powder coat suspension parts. A rattle can would be my choice. The ability to maintain the finish myself would outweigh any of the reasons to powder coat. You know those parts are going to get chipped. You don't even have to go off road to accomplish that. The same rocks that will chip your windshield are actively at work in greater force down closer to the pavement. Those chips are a bigger liability to powder coating than they are to solvent based paint. Manufacturers like powder coating because they avoid a host of EPA and employee health issues by avoiding solvents. That does not mean that powder coating is actually a superior product. It simply has economic benefits.
I know that is probably not the answer you were hoping for, but that is the answer I have from my own experience.
