Smart and Brown would provide an instant improvement in defense, passing, rebounding and size. I don't know if they would score the ball well enough, or shoot a high enough percentage to be effective. They'd need to be surrounded by better players at the 3, 4, and 5 than the C's currently have.
But if you examine IT coming off the bench you'll see the benefits. By coming off the bench he'd be playing, at times, against the opponents backup guards. He'd be also guarding backups, (a major factor). Teams would have little chance to adjust their defense to contain him. Just the fact of coming off the bench with a scoring weapon like IT will be of benefit by immediately changing the personality of the Celtic attack, causing your opponent to have to adjust.
Buy alas this is a pipe dream, the IT the media and fans have created, and that now has him believing he's a celebrity superstar. It would in fact cause internal, and external problems of major proportions.
Too bad, because IMO you will never go deep into the playoffs with IT as the starting PG, and main scorer. He'd be bounced around like he was inside a pinball machine.