Aye, it's not tough to layer them. In my game, I've got each starbase sitting at the very edge of the influence of the next in line, effectively ensuring that every freighter is getting a %120 bonus or more at all times. It can get pretty absurd!
yes it can. i'd build them by laying down one layer the way you described - every 8th parsec (7 parsecs between each SB). then i start on layer two, and SB between each of those (so that there are 3 parsecs between each SB), and then the third layer, another SB between each of those (1 parsec between each SB).
When you have managed to super-boost your trade routes, are you still competing with the other factions or have you effectively won the game already and just play on to accomplish some personal goals (like building the ultimate trade route)?
twice it was the latter, just trying to see how high i could get my trade. once it was something i set out to start early. i played korx and decided i wanted to make best friends with the korath and get them to do my dirty work. i got my freighters out as early as i could, started buildin SBs soon too. it was working well at first, except after i'd built about 30 SBs it became too expensive to keep building new ones because their initial cost grows exponentially.
however, i set the game up for just such a strategy: uncommon planets and habitables, common stars, on a huge map with 5 nonrandom painful AIs, very slow tech w/ no tech trading, only conquest as an option to win. with so few planets available, by the time i stopped building more SBs, my trade had become at least a third of my income IIRC. true, the pirates event would have boned me (not the peacekeepers though, since almost all of my trade route by then was in my own area of influence). after that i focused on buying planets from races the korath were killing off. i'd max out influence on those planets, and when the korath spored nearby planets they flipped almost immediately. at the same time, i focused on bringing my military tech up to snuff, and when the altarians were about the bite the dust i launched my attack on the korath. my economy plunged into negative income, but i'd saved up >20k and that lasted through the war.
as far as the personal goal, however, it was really cool to see a line of my influence grow pixel by pixel on the minimap right through the middle of the galaxy and about 3 intervening empires' influence.