The mast will be generally stiffer than the usual 400cm. The reinforcement at the boom may be misaligned also (lower than the boom) the mast extension may not fit in correctly due to decreased mast diameter, the bottom of the mast may also have less reinforcement if cut further up.if the sail has cams, they may behave differently due to decreased mast diameter? Apart from that.... it will be perfect 
Agreed, not worth it. General consensus here seemed to be cut no more than 10cm, and only with great consideration to sail match and your weight (not making it stiffer if you're a featherweight etc)
Always cut off the Bottom, not the top if you must cut. That way the bend curve will not be drastically changed and the stiffnes will not be dramatically increased.
And then, everything that Sparky said is spot on. 
Agree with sailquik. I cut a 430 mast to 420 to be used with Naish sails because I needed a bit more stiffness. Used it for very
long time after. Quick instructions LOL:
IF you need to cut > 30cm THEN buy_shorter_mast
ELSE
IF you need MORE stiffness THEN cut TOP
ELSE
IF you need to minimize the MORE stiffness effect THEN cut BOTTOM
all this shall go under a while loop not to cut more than 5-7cm at a time
and test/rig every time you cut.
Cut a spare 400 RDM powerex to a 370 and it works fine on a 4.0 and 4.5 Blade - if you cut the top the boom area remains the same and it makes F[_]ck all difference and works a **** load better than having mast sticking out the top - it might be wrong for all the keyboard experts but it works in the real world
www.peterman.dk/start-windsurfing-samlet-gb-dk01.htm
some info here is outdated.
this applies to the question
Some findings when measuring masts. Here is a link to some measurings of masts. Even if the measurings shown are selected, the figures are still mostly raw data - but here under you'll find some of the simliplified and generalized conclusions, you can make from the material: The mast neither changes stiffness nor bend curve, as the temperature changes. Even though you by means of the IMCS test can't find any difference, if it is 0 degrees celsius or 25 degrees celsius, of course you might speculate, if the mast perhaps gets a little "slower" in the lower temperatures - that is if the "flex response" is dampened.If you shorten the mast (from the bottom!), it gets softer and doesn't change its bend curve. Of course it's not wise to cut off so much that the mast foot/extension doesn't fit the mast - or that the boom is fixed above the extra strong boom area. And as indicated - never shorten the mast from the top.If you extend the mast it gets stiffer (most with Flex Top, a little less with Constant Curve and least with Hard Top masts), and it almost doesn't change bend curve. Of course you shall not extend the mast so much that you clamp on the boom below the strengthened area.When you try to find a way to calculate, what approximately happens to a mast that you extend, you'll have to decide what you'll understand with "extending". For instance, how long extension are we talking about (in numbers or relatively)? How much extension in the mast are we talking about (in numbers or relatively)? What is the stiffness of the mast to extend? What is the bend curve of the mast to extend? And so on and so forth ...If we make a fast decision and say that we are talking about a 30 cm extension of the mast by means of a 50 cm extension device (that's of course 20 cm up in the mast), then - based on numerous measurements - you can (very) roughly say that ...:All masts keep (approx.) their bend curves when they are extended.All masts become a little stiffer (according to the IMCS standard) when they are extended.Hard Top masts become relatively least stiffer (around 2-3%).Flex Top masts become relatively most stiffer (around 6-7%).Constant Curve masts become stiffened up between Hard Top and Flex Top masts (around 3-6%).This can roughly be "translated" in this way ...:If you extend a Hard Top mast with 30 cm, so that it reaches the length of the next mast in the line, the extended mast is supplied with 1/6 - 1/4 of the extra stiffness necessary to get on level with the next mast in line.If you extend a Constant Curve mast with 30 cm, so that it reaches the length of the next mast in the line, the extended mast is supplied with 1/4 - 1/2 of the extra stiffness necessary to get on level with the next mast in the line.If you extend a Flex Top mast with 30 cm, so that it reaches the length of the next mast in the line, the extended mast is supplied with 1/2 - 3/4 of the extra stiffness necessary to get on level with the next mast in the line.The more mast with or without extension that are measured, the more precise the ability to predict what happens as to change in stiffness - and the less likely it is that mentioned general rules have to be changed.If your mast has become "excentric", this doesn't change the bend curve significantly. However, as the mast now is a little "pre-bend", it equals it has grown a little softer.There is a tendency that the increasing softness that follow heavily use, are most noticeable in the areas that work the most - that is the top of the mast. But - you have to use the mast very much, before you face any problems in this respect.
