The turn is wont be as crisp extended, you will have to put more effort in the lean, and you will have more weight on the front tire.
That being said. With mine exteded I can hang with most of my buddies in the curves. I have a few really good riding friends that gain on me. But I think they would at stock length anyway. I use all of my rear tire. I'm scrubbed to the edges. At 8" over I was a little nervous with it. At 4-5" where it is now, I have no concerns. I press that bar and lean, and into the turn she dives. I'm also pretty low on ride height.
A set of good extensions will be more than strong enough. As long as you're not making 300HP they are fine.
With what you have in mind, I'd suggest looking at an adjustable arm 0-6". Nice thing there, is that all you do is adjust to the length you want and do the chain. With extensions, you will need to completely dissasemble and reassemble. Not a lot of work. but does add up over time.
I used Roaring Toys(I trust their CNC work), but have since gone to a full arm. Simply because I wanted to remove some weight, have an air tank for my shifter, and increased adjustability.
If you buy extensions just go with a reputable company.
Spencers is one of my preferred spots. Also look at ATC, Exoticycle, even look up Gix1300 on the boards. I used Roaring Toys this time due to availability and the fact that I've seen them turn out quality products.
You'll enjoy it either way, and with extensions you have the least investment.
I run my bike at 4-5 over and as low as I can get it. I run soupy's rear adjustable links, a spencers tripple tree, down .75" and a strap on the front.
When I run, I pull the strap down, get my two wrenches out of my hump and lower the rear. Get ready to go home, I get my tewo wrenches out and I release the strap. Makes for a nice happy medium.
For you, liking the twisties. I'd say get a strap and some radial mounts for the front, and get some adjustable turnbuckle style rear links. You can have very good straightline performance in 10-15min and be back ready for curves in 10-15 min.
As long as you don't stretch too far, youll still fly through the curves.
Also keep in mind, if the nose is lower than the tail, the bike will turn in faster.