IO sets

I would like to ask the more knowledgeable members a question about the IO sets,if I have slotted a power up with lvl 40 IO,s and then exempt down to lvl 30 would I still get a portion of the improvement that the IO gives i.e. if I get a + 20% damage by exempting down would I get+ 15% damage as I would have dropped down by 25% lvl wise.I hope this is clear enough for someone to give me an answer as I have no idea how these work.

Short answer: Yes, you get a proportional amount of your enhancements when exemplaring, but not by the "percentage of level" method you describe.

Long answer: There is a sliding scale of the percentage of effectiveness based on your "real" level and the level you are exemplared to. However, note that you may lose set bonuses or global bonuses if you exemplar more than 3 levels below the level of the IO. Better Explained on Paragon Wiki

Note that there is a difference between the "set bonuses" and the intrinsic bonuses - the bonuses that go with the actual power. There may be a better word for those. Set bonuses go away 3 levels below the level of the power.

Let's say you have four level 45 Thunderstrikes slotted: one Acc/Dam/Rech, one Dam/End/Rech, one Acc/Dam/End and one Dam/Rech. At level 45 you get almost exactly 20% for each part of a triple and 25% for each part of a double.

So you'd have 40% Acc, 20% End, 85% Dam and 65% Recharge if I'm doing my math right (and my math is sometimes right.) You would also have set bonuses: something like 2% to endurance recovery, 2.5% to En/Neg defense, and 7% to accuracy.

As long as you stay above level 42 everything works perfectly.

At level 40, you're not losing any "enhancement" (Because you're over level 32)- you've still got the 40% accuracy and so forth- but you lose all your set bonuses. No extra recovery, no defense, no extra accuracy.

At level 20, you still have no set bonuses. But you ALSO have all your intrinsic bonuses multiplied by .553. So that 40% accuracy becomes (40*.553) or ... umm, 22.6% accuracy. That 85% Damage becomes 47% or so.

A lot worse than your level 45 damage, but it's supposed to make you play sort of like a real level 20 character.

If you have, say, three level 40 and one level 45 Thunderstrikes, you would lose the "slot 4" bonus (accuracy) exemping to level 41, and the "slot 2" and "Slot 3" bonuses (recovery and defense) at level 36.

There are a lot of exceptions and odd cases, but this covers most of what you'll see.

Fulmens did a great job of explaining when the set bonuses vanish, and how the enhancement values are weakened when exemping. One thing to remember is that many special IOs have intrinsic powers that are not affected by exemping as long as you still have access to the power they're slotted in. For example, let's say you took Super Speed at level 14. Eventually when you were level 47 you dropped in a level 50 Celerity +Stealth IO. You would be able to exemp all the way down to level 14 and still keep the stealth effect.

Thankyou everyone for your imput I now understand how they work a bit better

The P-wiki has a very good detailed article on the subject.
There are several links within it that you may also want to follow.