besides the obvious reasons of slowing down guards  vs posts but are there other advantages?

I would think:
"+"  =  more steals, more fast breaks, and greater savings in stamina

"-"  =  more rebounds,  slower tempo, and greater ability to wear out a low stamina/depth roster


am I correct in thinking this?  or am I over analyzing it?  or are there any toher benefits for each?

I don't consider tempo at all with defensive setting. I consider where the strength of the opposing team lies, and where they do their scoring. I go +2 or +3 against teams that routinely shoot tons of 3's. -2 or -3 against teams that don't shoot from the perimeter or have major rebounding advantage. -1, 0, or 1 if your opponent is well-rounded. 

It's also important to self-scout. If you have a team in which your perimeter defenders are already excellent, you might sag inside a little on defense if your interior defenders aren't as strong. 

There are definitely benefits to each depending on matchups. 

Sometimes you can also do the "sag and double"

so on my IOWA squad I only had one guy worthy of taking a 3 but my other guards had pretty decent ATH and SPD so they were pretty good at still scoring inside the arc.  if you were playing me would you go to a "-3" because of where my guards score? 

I would look at how often you take 3's as a team and base it on that. If I had a man to man and your 3 point shooter was not that great at BH and P, I would double team him and move to -3 or more. If he is the only guy that ever took a 3, I might double team him and go to -5.

Fouls are also impacted by + and - settings ...

All other things being equal, you will generally foul more when playing "-" and you will foul less when playing "+".  The bigger the value, the more impact (so foul the most at "-5" and the least at "+5").

The only time I allow fouls to really come into play is if my defense is Press (you already foul  alot in Press, so I normally do not do a large "-" if I am running Press) ... or ... if my team is very young (if my IQ's are low then I might not play a largely "-" defense). 

Other than those instances, I normally play my defense + or - based on 2 things:

1.  The Ratio of (3pt Attempts)/(FG Attempts) ... with 1/3 being what I would consider a 0 defense. Here is the current chart I would use:

Defensive Positioning
Recommended	Basis	Max	Min
Positioning
+3	NA	100.00%	48.00%
+2	40.92%	48.00%	38.81%
+1	36.71%	38.81%	35.13%
0	33.56%	35.13%	31.67%
-1	29.78%	31.67%	25.48%
-2	21.19%	25.48%	18.86%
-3	16.54%	18.86%	8.27%
-4	NA	8.27%	0.00%

Note:  That is for M2M .. it would need adjustment for Press or Zone.

2.  The % of points scored at the 1,2 positions compared to 40% and the points scored at the 4,5 positions compared to 40%.  (In a perfectly balanced team, 40% of points would come from 1,2 ... 20% from 3, 40% from 4,5).  So if a team had 20% from 1,2 and 60% from 4,5 (that would leave 20% from the 3) ... then my equation is:

B is Bigs (positions 4,5)
G is Guards (positions 1,2)

((40-B)-(40-G))/20
((40-60)-(40-20))/20
(-20-20)/20
-40/20
=-2 (so -2)

70% from guards, 10% from SF and 20% from Bigs would be:

((40-20)-(40-70))/20
(20-(-30))/20
50/20
=2.5 (so +2 or +3)

3.  I would average in some way those 2 numbers (the part from the ratio of 3 pot shots to 2 point shots and the part from points scored by bigs or guards) to determine which I would play.  I would also usually limit my defense to the range of +1 to -1 if playing press regardless of the calculation (If I was running press and the calculation was -3, I would play -1).  I would also subtract 2 from the value if I am playing 3-2 Zone or add 1 if playing 2-3 zone. (if my calculation was +2, I would play a 0 if I was running the 3-2 Zone or +3 if playing 2-3 Zone).

I am not saying this is the best way come up with a number ... just telling you what I do.

thx for the insight HUGHES