you may be right and I may never know because I don't have the tools to test the differences. However, I do know that the +20 magic penetration from the guise accounts for something like 14% increase in damage(varies with MR of your target).

By the time you are say.. level 12 with Will of the Ancients your half-charged rank 5 howling gale will do around 400 damage without the +magic penetration from runes/guise/masteries. So, 14% from 400 is around 56 extra penetrating magic damage. It is even better for fully ranked Zephyr since it has more base damage and +AP percentage.

Since the guise itself adds 25 AP we can say that its actual bonus with its magic penetration is 25+(56/0.75)=99 AP. This theoretical AP only relates to the damage of Q and W (of course) and not to +heal and shield absorbtion of the other two spells. It gets even better since I did not account for Executioner mastery and the further increase in AP from rabadon's deathcap(and auras/buffs of your teammates). In the end, the theoretical increase in AP for Q and W from the guise could be anywhere from 160+ AP by the time you are level 18.

Unless my calculations are wrong I still believe that 1485 gold for this item is totally worth it. The magic penetration seems just too precious to pass on(at least theoretically). You can try calculating it yourself if you feel nerdy. I only used those two sites and a calculator:
http://leagueoflegends.wikia.com/wiki/Magic_penetration
http://leagueoflegends.wikia.com/wiki/Magic_resistance

glad you liked this guide. I might clear up those points in the guide later in time but for now I don't have much time so anyone who wants to copy paste it is free to do so.

oh yeah, i dont really buy the hauntin guise so i go strate 4 morellos since hauntin guise doesnt really give u that much hp/ so irather go for sum other cheap support or ap item

hey dude i tried out ur build and comin from a semi amateur @ LoL, it was fun, sure my first game sucked donkey ballz but that was due to my teammates, after that i saved countless of lives w/ my shield+ult and even 1v1ed tryndamere and win :3 thx for the awesome guide

because 1) you get push back with rank 1; rank 2 and 3 add +heal only 2) each extra rank adds only 40 +heal(-->80 +heal at rank 3). But, I'm still open to debate on these on two points. I find it more useful to invest the two extra ranks into eye of the storm which translates into +80 damage absorbtion and +19 AD buff. This +80 damage absorbtion of the shield is of more siginificance because with +40% cd readuction you can cast it every 6 seconds unlike the Monsoon's minute and a half cooldown.

um why do u lvl up monsoon last?

i had kinda came across most of this myself, using janna as a dominion beast, but this out the finishing touches on it..thanks