RE: 2006 GO 170 vs 185
I think you have a good plan.
Since it sounds like you will have enough windspeed to be planing on the GO 170 virtually all the time (with a least an 8.5 m2 rig) it's best to get the "time on the water" now.
The GO 170 will actually give you a better overall high wind range.
What you lose on the bottom end of the spectrum (in < 10 knots)
you will gain in the 18-22 knot range with smaller (5.5-6.5 m2 rigs).
Hope this helps,