Yes, it should be fine to prune it like that. JM's allowed to grow in this manner with multiple leaders usually have an attractive internal branch structure. Judging by your zone the tree has probably recently shed its leaves, in which case now will be a good time to do the pruning.

(The usual advice is make a clean cut with sterilized tools etc. but you probably know that already.)

I agree, the reduction should be fine as long as the root system is large enough and the tree appears to be in good health (has not had any pest problems etc.. recently). I would recommend buying some tree wound sealant, or at the least using some Vaseline to cover the wound. Open wounds (especially larger ones) can cause an opening for fungus and insects to harbor in the tree regardless of season or location. Also as mafle stated make sure the pruning utensil is sterilized properly. Rubbing down the knife/cutter with alcohol prior to making the cut should work fine (for the same reason a doctor would sterilize a tool between patients).

If you would be interested you might want to look into how to do an air-layering as well. You could end up with an extra tree. Its fairly easy for beginners to accomplish and no specialized tools are involved( If you were to do so you would do it in the spring after the tree grows in its new leaves). If worse comes to worse and it doesn't work out and the branch does not root no big deal, you were going to remove the limb anyways. This is of coarse assuming the limb isn't massive. Just a thought.

-Mike

I can't believe what I'm reading here.
Advising someone to top their tree then to apply vaseline to the wound?!

I would advise not cutting off your central leader and not applying any kind of "wound sealer". Call me silly I guess.

Japanese maples often grow naturally with multiple stems. Not having a central leader is no big deal.

I agree that removing the 'leader' is a personal choice - when mature, J. maples (and most other deciduous shade trees) do not have a defined leader. However, I also agree that applying any type of sealer to a pruning cut is not recommended and has been discouraged for a number of years. Sealing or painting a pruning cut has a tendency to enclose any types of pathogens and excessive moisture rather than prevent them from entering. A clean cut using a sterilized tool, preferrably on an angle, and on a relatively dry day is all that is required. The tree is quite capable of compartmentalizing and "sealing" the cut itself.

Thanks for your advice folks. The tree is healthy and one of those branches actually grows higher than the 'leader'. I did some other pruning and thought it might be too much for this year but will plan on removing it next year. I might try "air-layering" also. Some of my other maples don't seem to like the wind and salt air here, but this maple is doing very well in that respect. Thanks again.

Leave a little room for Die back (about an inch or two)... And Never Never Never use any sealant; If your tools are sharp, and clean enough, and as long as you don't damage the cambium, your tree will be fine.