80 IV Lapras FB/IB for attack only should I power up past level 30?

Depends on whether you want to use the dust or not. My view is that Lapras is a good attacker/prestiger into Dragonite, but otherwise not very useful. I have one powered up to 2300, used for attacking Dragonites, and another at 1400, used for prestiging.

I don't see a reason to level any mon past level 30.
Specially attackers.

I'm thinking about this matter myself and I incline to, but I have more candies than you (I hatched 4 -four- in last 5 weeks).
With my FB/B just powered up to lvl 30 last evening I attacked a gym with 8 consecutive Dragonites, all 3100+.
At level 30 I managed to kill 2 and 3/4 from the 3rd and I'm thinking that might worth getting up to 32 or 33 in order to manage that. I had similar results both by dodging all (trying at least :) ) and by dodging only charge moves. That is why I'm seriously considering the power up once I will theoretically prove (totoro's spreadsheet :) ) that he can take 3 3100-3300 Dragonites.
With lvl 30 FB/B and FB/A Cloyster I was able to kill max 2 Dragonites, but most of the time around 1 and 3/4 of second.
So, to answer your question, depends on how many Dragonites you want to kill in a row.

Attackers and prestigers almost never need to be powered up. When anyone asks whether they should power up this or that for attacking/prestiging, 99 times out of 100 the answer is no.

Save your dust and candies for defenders, especially Blissey, Snorlax, and those that can go up to absurdly high CP.