My friend has 2 14"L 6x8" oval straight through magnaflows.. The inlet on them is 2.75".
Would there be any issues with exhaust flow if i step up my stock 2.25" with adapter couplings to 2.75" right before the mufflers?

To be clear, i would leave the stock piping to the axle, and step up only for the axle back.

 

no, but when you modify exhaust...you're effectively experimenting. Go fourth and experiment.

meaning; no one knows what will happen. I say it will be fine, you wont run into any trouble. cuz people change out axle backs all the time