I've never used a 490 cm mast that was less than 75% carbon so it's difficult for me to imagine how much a 490 45% mast must weigh.
Here's the specs from the Tush website for the 45% Freewave mast:
Length : 490cm
Carbon Content :45%
Construction: wet lay-up
Weight; 3.00 KG. (6.6 lbs.).
The Tush Freeride 490 cm mast (75% carbon) weighs 2.35 Kg (5.1 lbs).
The Tush Pro Series 490 Cm Mast (100% carbon) weighs 1.9 Kg (4.2 lbs).
So, the 45% mast weighs 57% more than the 100% mast.
May not seem like much, only 1.1 Kg (2.4 lbs) but when you get the additional weight
out near the end of a 490 cm (16.08 ft.) mast, the additional weight really adds up.
If you are a big strong fellow, it might not matter too much to you, but if you are small to midsize and not super muscular, it will matter a great deal.
As to which sail will plane earlier, the 8.6 m2 will definitely plane earlier.
Is the 45% 490 mast recommended for this sail?
The IMCS looks about right, so the 8.5 should rig on the heavy mast pretty well and give you fairly good peformance.
So, yes, the 8.5 will definitely plane earlier, if you can deal with the tremendous additional weight.
If you can't deal with pumping the longer, heavier mast, to get going really early, you might have to just not sail on 8.5 m2 days, or slog on your 7.0 m2 freeride rig.
To tell you the truth, I was not aware that anyone even made 490 masts in 45%.
Hope this helps,