Wow... From what I can see NA tuning is a bigger pain than FI tuning... or maybe all the turbo guys have already done all the trial-and-error. I dunno.
Got nothing to do with "trial-and-error" on either side of the fence... both methods can be predictably built from an on paper design to an exceedingly high level of accuracy.... its not a black art if you take the time to understand what it is you want to do, and what it takes to get there.
The reason why FI is taken over NA, is that at a certain point (usually the point where you need to rebuild the engine), FI becomes more economical.
For example, to get an FSDE to push 200whp NA, would require some insane custom conrods (to withstand the insane forces involved at the kind of RPM required - anyone who says rod strength is all about power holding is talking out their ass.....the forces involved in moving a rod up and down at the speeds we're talking about are mind boggling), possible destroking of the engine (to help with the rod:stroke ratio, which will assist in cutting down piston accelleration), isnane fuel (so the fuel burn isn't outrun by the pistons), overboring (to get the pumping capacity of the engine back up after destroking it), rediculous amounts of head work, insanely high compresson....a gearbox that can put up with the RPM (single piece synchros don't really like working at 9000+ rpm).... manifolds...the list goes on and on.
To achieve the same horsepower on a turbo build requires much less work. In fact, just to give you an idea of how insane a 9000+ rpm build 200hp NA engine is....using some rough formula's available on the garrett website, our 9000 rpm 200hp NA motor, equates to a boosted motor, at a mere 1bar of boost (and assuming the same kind of work to the engine as the 200hp engine, so about 110% VE before boost), you're looking at around 2x the power at the wheels.... *and thats only on 1 bar of boost*... you ratchet that up to around 28psi (a little under 2bar)... and you're looking at around 575hp...
Now, i wont go through the equations, i've made a lot of assumptions...and its intended to just be a rough guide.... but essentially building a 200hp NA FSDE, is like building a 575hp turbo engine - with all the cost and hard work to match, with the same sort of drivability to boot.... using those numbers as a GUIDE - you can begin to see just how economical, and practical it is to build a 320 to 350hp turbo set up....
*thats* the reason why turbo charging is so attractive. Yes theres a high cost of entry, but compared to doing an NA build that goes like a rat up a drain pipe, its really quite a cheap way out......