It's important to remember that these folks in here are replacing much older, likely blown, suspension components. Kinda and apples to oranges comparison, for you.
That said, which springs did you use on the 5100s?
And I hope you mean a top plate spacer, and not coil. A coil spacer in search of a better ride will only disappoint.
I'm glad someone with some credibility said this. It wouldn't mean as much coming from me but your posts on this thread are spot on for the OP. In my opinion of course, but comfort, as most describe it is going to be best with the OEM setup. Sporty is another word with another meaning and that's what you'll get with the other options.
Heck, even the Tacoma 5100s will be slightly more sporty than the OEM and still less sporty than the Tundra 5100s. I paired the Tundra 5100s with the Tundra TRD coils and I can say the the ride is not as sure-footed as I would like it. Sure, it corners a lot better, but it's a 4Runner, not a coupe. It gets hoppy on the smaller bumps and especially at high speeds with slight high and low spots i.e on the freeway.
Too many times you see reviews of modded or pieced together suspension system and people say, "I like the ride, or it rides good". That doesn't say much.
I have 906s in the back with OEM shocks and am satisfied with the rear end smoothness. A 4th or 5th gen shock in the back might better though for proper flex etc.
With 99 talls with Tacoma or Tundra 5100s (Tundras being stiffer) in the front you'll get adjustability and with 906s and air bags in the back you won't have to worry about sagging when you decide to tow or carry every now and again.
This is all opinion of course but I was in your position not long ago and after trying out one of the highest regarded setups on this side I can confidently say that unless you have weight in the front or want some gnarly lift in the future try something besides the TRD coils and 5100s. I know this wasn't an option you were discussing here, but in case it was in the back of your mind.
I think you're on the right track. The info that A.Wilson has described here is gold if you're looking for a great ride without any sag in the rear.
Don't forget to take into account how much you paid for this advice.
Matt