I'm having the same debate. I finally picked up the LFR version of t15 helm, replacing the t14 helm I had been using. With the normal version t15 gloves and legs, that puts me 3/4 ... and I've had chest and shoulder tier tokens in my bank for months. However, I'm quite partial to the itemization of the chest and shoulders I'm wearing now.
If you're not pushing any progression (or at least anything that's healer-dependent), it's probably acceptable to hold out for your helm.
No, definitely not, but the helm was an upgrade. If I get a 522 helm soon, I'd be happy to drop that.
I scratched out notes based on replacing the 522 Dinomancy Spaulders with LFR t15 shoulders. Assuming 2/2 upgrade to both, works out to just under 250 loss of Int, ~200 loss of spirit, 600 loss of haste (which I can gain back by changing around my Crit re-forges to Haste instead of Spirit/Mastery), but about 700 gain in Mastery, plus the increased RJ ticks.
If nothing else, I'll see how things look this week. If it's not worth it, I can always put back on my Dinomancy shoulders (though I'd miss them -- first item in ToT I ever received). And if a superior helm lands in my hand (so many runes thrown at Megs), I'll re-evaluate.