Is your Macbook Pro within the 1 year limited warranty period? if yes, you can still bring it to an Apple Authorised Service Provider. It only mean that you have 2 different warranties overlapping.
Check warranty status @
https://selfsolve.apple.com/agreementWarrantyDynamic.do
If it's still covered under Apple's 1 year limited Warranty, I would recommend you to call AppleCare @ 800-186-1087.
Inform the AppleCare advisor that you have done an erase and install and you are still facing sluggish/performance issue after that.
You also need to point out that you DID NOT restore your data after doing an erase and install because that can only mean that we have not isolated it to hardware fault.
Most likely the AppleCare advisor will get you to bring it in to an AASP for further Hardware diagnostics.
Why I say to call AppleCare first? because if you were to go to an AASP without speaking to Apple, they might want to charge a diagnostic fee if they couldn't find any hardware fault.
But if you were referred by Apple, it's a slightly different story - of course you want to make sure that you gained agreement with the advisor that you should not be charge for anything upon visiting the AASP.