i play on ps3 and am considering a laser build, will I see serious lag due to using lasers (ash piles). please answer if you have used ew extensively, I'd rather not go on speculation.

Depends on how much you play I suppose, I just spent around 100 hours on my 3 EW characters on the 360 and I had no issue at all with lag from piles.

I try to role play, so I'd say significant time. but good to know from someones experience. I tried to think of areas that have respawning and I don't see myself traversing through such areas several time during a play through

Well, I raided Black Mountain on my first EW character, lots of times. And the goo/ash piles started stacking up, but I never experienced any lag issues from it.
I did try to loot one pile who's original NPC had respawned though and the game crashed.
So don't loot piles until you're sure you've killed everyone.

My main character Ruin uses EW and Guns exclusively. Though he uses EW more often. After 742 hours on him I'v rune into the same Ash Pile once or twice, and have not experienced excessive lag because I'm crit happy (Quite literally so, Ruin has a 57% crit rate or so with Laser Weapons). I am on the 360, however, though my brother plays on the PS3 and reports the same thing.

I made a crit-build and by the end I was literally ashpiling all the enemies, I saw no lag later, even with multiplas rifle Arcade Gannon and upgraded weapon ED-E, we walked arround turning everything into ash.

Of course I was playing on a super computer...

I am currently playing an Energy Weapons only Indie character on PS3.

I had awful lag too until I deleted the files of the games I have beaten and/or no longer play.

I got very little lag while playing on it.