I have no doubt when you know what you're doing, you can assemble your own the ultimate kit and make significant gains with all the right parts. And it does sound like you know what you're doing. For my P4, I just purchased the Itellite antenna and necessary internal cables they offer online in a kit form. There's another kit that starts with the antenna and adds two Sunhans amps as well. But I was initially drawn to the cleaner look of just the antenna. After I installed the antenna, I noticed a significant gain in distance when I was flying in an open field (no obstructions). But when I attempted to fly around my own neighborhood, there was little to no gain in distance. The signal was lost as soon as any of the neighboring structures (homes) blocked the LOS between RC and drone. From what I've seen on YouTube, I knew I could do better. I've seen several videos posted where the drone pilot has take off from their backyard or front yard, and have flown out to much greater distances. In almost any urban area, as soon as you're a block way from home base, you're bound to have a building obstructing your LOS, unless you take it straight down the street. But that's not how these pilots are flying them. They're taking off in a rather tight area of homes and other buildings, and flying considerable distances to reach their downtown areas. They're taking off from their street and there are several obstacles between them and their drone during the flight. I'm not as interested in standing in an open field just to earn the ability to say I flew 8+ miles away. I'd prefer to fly around the home area and not be concerned with losing my signal two blocks away. I'm flying at altitudes of 200, and have tried taking it up to 300 and 400. My signal still drops off pretty quick.
To be fair, I have yet to drive to an open field and try this new configuration (since adding the amps) for a fairer comparison. But I can say that around my own neighborhood, I see no improvement and am losing my signal at about the same or close to the same distance as I was before adding the amps. Sorry for the long post. It just seems no matter what I modify, I can't achieve the distance others are achieving from their backyards. I'm getting a Mavic Pro soon and have decided to keep everything stock. I'll see how that goes for me.