Had a go today of a 7.0m,rigged on a 430 X9,twisted off to second batten down,lots of negative outhaul.Wind 5-15 knots.
Was amazed by how soft and incredibly light this sail is.
Also with such a short luff 448cm and boom 200cm,it felt real easy to gybe as it felt smaller.
However, got the feeling that due to the super tight leech this sail could overpower very quickly.
Sounds like you may have not had a lot of down hall. You probably had it set up perfect for the light winds but the MK4 should downhall a lot more (loose leach to the panel above the boom) I found with the MK4 that the wind range was quite good but downhalling to the right amount was crutial, to be at it's best. I normally went around 1-2cm off max.
I use the 7.8 in up to about 25 knots of wind altho its most comfortable at about 18-20 - with a lot of downhaul the sail is very well behaved. once you have these sails locked into the raked back position they become super stable even in big gusts.
For the bigger sizes use an adjustable outhaul - you can get a really big performace difference with even a CM more or less out haul. As you tighten the outhaul you can feel the center of effort move foward in the sail making it easier to control at the expense of some power but honestly in 20+ winds you already have more power than you need :)
These sails are designed to be sailed overpowerd :)
Yep definitely under down hauled
Pull it down so there is twist to the just above the batten above the boom
7.0m sails rigging on 430 masts? WHAT SORCERY IS THIS?
used to be no self-respecting large race sail would consider rigging on anything less than a 460.
also: you kids - get off my lawn!
Shane, the blade loft 7m has rigged on a 430 for a few years now, so its not that unusual imo.
Offtopic, guys with an adjustable outhaul setup, just say you are cruising along with ya harness lines in perfect optimum balance, then a big gust comes along so you reel the outhaul in 2cm on the go. Your harness lines would then be in a bad position leaving you more front handed. Are you also adjusting your harness lines to compensate or whats the go?
Darkplague.... having outhaul off means you are sailing downwind slightly and if you go upwind with outhaul off in a gust the back hand pressure increases.This means you should set up harness line position for up wind outhaul on.
When you go downwind the harness lines are good with outhaul off and prevents you from over sheeting.
So if you set up harness lines for up wind and outhaul off then they will be not optimal when you sail downwind.
Try having lines a hand width apart and will give you the range you need when going downwind.
Hope that made sense
Yer makes sense.
I really meant in just normal sailing, across the wind. Just curious.
Tried the adjustable outhaul on my big sails at once stage. Got it working real smooth on 5:1 gearing. Took it off as I prefer to keep it simple these days, also didnt like how it increased your boom length by the extra shackle near the clew hole. Then again im not a gps guy, so no real need for it, just like trying things out!! 
Never used adjustable outhaul,adds weight,a nuisance as rope gets in your way.
Besides, never had the need for it.Rig with negative outhaul all the time unless 25+.
Tune power by sail size selection and downhaul.
You can go upwind great/better with negative outhaul,especially in the lulls.
Can't sail properly with positive or neutral outhaul,just what you get used to.
