I had that problem with a x-pipe once I installed my ESS G3, I had 2.75 pips with flow masters and I had drone and raspyness with 2m Fabrication Headeres, I installed MBRP street along with H-pipe did away with the drone and raspyness. A exhaust is 3' no cats sounds great when you get on it. NO...
I run the BMR BREAD AND BUTTER KIT WITH EIBACH LOWERING SPRINGS AND VIKING DOUBLE ADJUSTABLE INVERTED SHOCKS ON THE REAR, IT HADLES GREAT.I MAKE 671WHP, 2K15 GT PREMIUM WITH 3.15 AND A ESS G3 WITH 120MM PULLY
Before ypu spend money on replacing coils mark the one that is giving you problems and change it and see if the problem follows and that would let you know thats your problem
I went with the BMR bread and butter kit with Eibach lowering springs and viking inverted double adjustable. Oh yea it's a convertible and it's heavier
Im currently running Ess with 8-9 LBS I run R2s with no issues im 6r80 also and 3.15s, you might have to much air in tires. I run the BMR Bread and Butter kit, with Eibach lowing spring and Viking inverted double adjustables.
A few thing I would ask is about is a warranty and was the Super Charger installed by Ford. They will allow you to take it to a outside mechanic, put it on a lift check suspension parts and rear CV Shafts, you will be able to see if it's been abused.
I have a 15 vert i was running, 2.25 stock pipes going to a 3' H pipe going to 3' flow masters and it droned like crazy, went to MBRP Street with H pipe no drone
If you plan to go e85 or make any other(CAI, manifold, forced induction) mods wait until you do those before you get a tune so you won't have to pay more for a return. Your car will run fine with stock tune and headers. Im tuned by Rob Shoemaker, hope this helps.
I have the MBRP STREET as I remember the mid pipe was slightly longer on the driver's side. I had to cut my other one off they boger welded it, and it had stock pipes and 3in pipe and ait droned like crazy...MBRP street no droning no rapping all same size sounds great.