MJS,
I have a 150 genoa and a 105 jib, both on an original Harken Mk1 furler. They are both Quantum Fusion M laminate sails. They have low tech polyester threads (not kevlar, carbon or spectra) on a mylar membrane. They do not stretch compared to Dacron. Both sails are performance designed, full hoist, deck sweeping, with lightweight UV Dacron sun shield. I don't ever plan to partially furl them, although I have done it for both.
I can get sailing in 2 knots of wind with the 150 and my headstay set loose for ~ 10 inches of sag (backstay totally off). By 5 knots, I'm powered up and passing every heavy cruiser on the Bay, particularly those with shoal keels and furling mainsails. At 10 knots, I'm getting near my max heal upwind (the heavy cruisers are smiling and sailing now). At 12 knots, I'm maxed on the 150 and starting to depower the sailplan. I can push the 150 to 14 knots if I flatten the main by bending the mast with max hydraulic backstay (this also takes all the sag out of the headstay). I also move the jib lead aft to twist the top of the genoa and spill some wind. I'm healing at 15 - 18 degrees with no crew on the rail. Weather helm due to the heal angle is starting to slow me down due to rudder drag.
I try to never sail my genoa partially furled because the shape is horrible (I don't have a foam luff and the shape is deep for light air). My sailmaker built it with enough fiber for 17 knots because he knows I've got no chance of changing to my 105 in 14 knots even though I said I would. A good sailmaker asks you how you sail and knows when you are lying

.
My 105 gets sailing at 6 - 8 knots and is good at 10. It's fully powered up at 15 knots and maxed at 18. My 105 just fits ahead of the spreaders, has the vertical battens that Loren describes and also inhaulers that pull the clew inboard for a tighter sheeting angle and more low end power. The inhaulers were recommended by my sailmaker to boost low end power and reduce the need for sail changes which are a royal pain with the sail sizes on the E36RH.
Bill,
What size sail are you dealing with? Generally, a +/- 130 is the largest size genoa that sailmakers will recommend for partial furling. Newer sails have lots of features to make partial furling work better. An older Dacron sail will have stretched out of shape and will get more stretched as the wind increases.
A major decision is do you want to go with two sails and the cost, storage and changing hassles or just a single sail and partially furl it. Our area penalizes the single 130 more than others because we have a lot of 0 - 8 knot wind days where a 150 is optimal and a 130 is underpowered. Having the second reef is not going to help a 150 or 130 function over 20 knots. I'd have my 105 and a single reef at 20 kts. This is where a good sailmaker will ask how you use your boat and figure out what set of sails makes the most sense.
Mark