Is my exeggutor worth powering up?

Honestly, tough call. 95% IV's hard to pass up.

While it's a 'middling' moveset for him (not the best for offense or defense) it can certainly be used for both. Also, the way gym defending is currently, CP is really all that matters. Even if a lower CP Pokemon is 'harder' to beat, if it's at the bottom of a gym it will get knocked out in 20 minutes.

However, looking at your stardust, I'd say no. It's going to take you probably 100k dust to get that beast up to level 30. Be patient and don't power anyone up until at least 28+

You'll kick yourself if you find one in the wild later that's lvl 28-30 which will save you the dust. It's generally best to hold off as long as possible until you see a need to power something up

Nothing worse than having a high IV and imperfect moveset, it would feel better the other way around. But patches may change things. I wouldn't do it, would just be irritating everytime I had to put more dust into him

Your Exeggutor is a perfect gym trainer!
And the good news is that you don't actually need to power it up any further. He's exceptional water type counter. Even if confusion is not rated as a top attacking fast move, you have to consider that it fits perfectly in the middle of two opponent's water gun attacks, followed by a dodge. This guy can win against two 2x its CP vaporeons (or other water gun users) in a row. That means 2000 prestige with a single potion!