Suppose a toon just dinged 115 and has completed Hero of EW and Hero of GD. I'm guessing that completing Hero of Velks and Hero of Kael (and so also Paragon of ToV) would NOT be more experience than all of level 115. I'm actually hoping it would not (I'll probably ding 115 without those two missions done). Since we're not getting niceties like Glyph of Stored Experience, I don't want to waste the experience from those missions.
On a related note, I feel like I went from around 9% into level 115, to about 40% into level 115 when I finished Kael Savior and Partisan (and the expansion). I was worried about over-capping experience as well, but it wasn't even close.
I would not recommend not completing some missions as stored exp for future expansions. You will be missing out on too much achievement rewards like the trophy if you leave them undone.