Modric playing at SPL would definitely be good for them if they can make him play. Look at Ronaldo and look at Benzema, one of them plays his heart out and scores, just had a hattrick and he is putting 100% of himself to the field, the other ran away for a while and hasn't scored all that much.

Modric is not a scorer like those two, so it Is not clear indicator on how he plays, but basically if he doesn't get injured and plays almost all games then yeah he will be good. They can pay him 100+ million a year, he would be famous enough to do that, but at the same time it wouldn't be easy to convince him neither. Just because he is there, doesn't mean that he would be willing to play, that Is why it is a risky move to pay that much to him.