Nothing good about increasing the shock length my friend apart from road clearance and probably be a bit more comfier as there's more suspension travel. Negatives there's a lot. Main one being affecting the suspension geometry which has an adverse effect on all the suspension related components like bushes links joint anti roll bar etc as you want to raise the rear suspension there's not all of them components involved but still there is some bushes etc anyway in desi engineering we use some aluminium blocks between the spring and the body to increase its length which is called packing.
Dnt forget to have your wheel alignment done after doing so.