I'm buying lenovo Y510p which has different models. Both of the models have an inbuilt GT 755M. But one of the model has GT 750M and a dvd drive in the ultrabay. Another model has GT 755M SLI inbuilt and a GT 755M in ultrabay. The question is if I buy the model with dvd drive as I need dvd drive and buy an extra GPU GT 755M which they are providing on sale for $99. Will both the GPU perform normally or I will have to buy SLI only? Will it be a great difference to go for SLI?