Getting into Mumbai and back is likely risky in 8 hours; especially in the rush hour. You could go to Powai; its a bit western has shops; restaurants and you can even choose between a costa and a starbucks; there are a fair amount stores / bars and restaurants which will be nicer than the airport – and let you burn off your last rupees before leaving.
On the way there you will go past Powai lake which is fairly peaceful if you want a walk; bar the traffic noise. The Renaissance hotel is a reasonable place to spend some time on the other side of the lake it has a particularly nice italian restaurant (for india don’t get me wrong); the buffet restaurant is mediocre at best. There is more choice in powai – but it will be busier. Powai is the other side of the lake.
You should be able get taxis to either powai or renaissance without issue.Cant answer about the ibis I’m afraid; usually get a hotel car to the renaissance (or sometimes holiday inn if renaissance is being crazy price when i arrive.
quick edit; its worth having some time in the airport if you like art, it has a great and varied collection