Thanks again everyone for all the helpful information. I've been combining that with studying and researching, changing my opinion a little at a time.
Couple important things that have shaken out of this:
1) The KL 505v is probably not going to be suitable or helpful for future ham use, and possibly not legal either if it does not have the proepr FCC blessings. This suggests that scrounging up 35% more cash for a 505v over a 503 is probably not worth it.
2) The gain of any 300w rated amp (like the 503 or the 505v) is not actually super significantly higher than a 100w rated amp. There is a really steep curve of diminishing returns, dollar for dollar and watt for watt, once you get past the first 100 watts.
My research lead to me creating the following chart, which is based on borrowed information and rough math so I'm not saying its scientifically accurate, but its probably accurate "in general":
Figuring out how to spend limited hobby dollars wisely gets a little convoluted because of weird factors like how 88 radio will sell a package of two 203 amps for a discount vs buying one, and they seem to have an additional sale that just comes and goes like DX on the wind, but assuming the best case scenarios I have seen...
A (patient) person can catch the right deal at 88 radio and buy a pair of 100w rated 203P for $69.50 each. Worst case $90 each. Side note, if their additional sale is running I'd say its worth it to spring for the pair of 203
Ps but if the sale on the 203Ps is not running then I'd just go with the pair of 203 without the preamp.
Compare those figures with the jump up to a 503 for $210.
A $120-$140 increase for maybe not even 1 whole S-Unit increase on the other end. That is not what I call great value. It's a 3-5 DB gain which is not nothing, but its sure hard to justify.
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Last night I made a run up the highway to a nearby town 8 miles away to drop off recycling and buy some groceries. I used the errand as an opportunity to do a little experiment. My wife stayed at home on our base station (the 980SSB on the CLR-2), I set the receive gain at about 3/4 and showed her the squelch adjustment. Then I got in my car using a cheap tram 703 (2' mag mount centerload) on my $5 ebay special, a vintage General Electric "Help" 3-5920A radio. I threw it a bone by at least using an external power supply for it rather than using the car 12v, but even still if you know that radio at all... it doesn't get much more spartan and bare bones than that.
Anyways, off I went on my errand. The route is representative of some pretty much worst-case scenarios for interference. Several radio and cell towers, and a whole 'nother town, were between me and the base station by the time I reached my furthest point, 8 miles away.
At 8 miles I could still hear my wife's voice in the static but it was not really understandable. Biggest problem there is she was speaking like a person whispering at church.. It seemed like she could see an increase in static when I tried to reply but she could not discern my voice.
She could pick me up again once I was back within 6 miles and away from buildings and street lights and other interference. At 3 miles we could talk pretty clearly even through I was smack in the middle of a grocery store parking lot in a populated town, and under 2.5 miles static was non-existent.
Most of the local AM stuff (4-5 other stations) that I want to reach consistently are within this 3-6 mile radius of my base, so my takeaway was that throwing 100w 203's into the mix would be a big help, but springing for 300w products would not help much more despite the huge cost increase.
Originally I figured just maxing out what my antenna could do (300w) was worth the extra $$ but the math I found doesn't back that up. Add to that the non-usefulness of the amp investment for anything other than 11m CB, and the 300w products just don't make sense anymore for the money.
Seem like I'm on the right track now or am I off the rails?