I'd be surprised if Porcello's HR rate didn't drop a little moving to the NL.
Yes, you'd think that all else equal a guy like Porcello should allow 2 or 3 fewer HR's when coming over. He's typically allowed about 18 HR's per season, which averages to 2 HR's per each spot in the AL lineup. Replace a DH with a pitcher, and with only 8 instead of 9 hitters in a lineup that should drop to 16. Perhaps the drop should be a hair more, since DH's always have some power, whereas the average HR's output of regular hitters is skewed/depressed a bit by the Barneys of the game. So I'd think a reduction of around 10-15% in HR allowance would seem likely.
That said, I think the perception of impact is sometimes inflated a bit. I think sometimes there is the perception that all the AL parks are HR-launching pads, Baltimore and Texas and Boston and Yankees, and that AL lineups are packed with sluggers. But that is certainly NOT the case in the AL central. Detroit, where Porcello pitches half his games, has been notorious as an anti-HR field. They play a bunch of games against Minnesota, which has an extreme anti-HR field (by reputation, at least), and has an absolutely powerless lineup. Kansas City is not a power field either. In Porcello's division, there are just a whole bunch of low-HR stadiums and power-deficient lineups.
So I'd guess that if Porcello came to the Cubs, removal of the DH should take 2 or 3 away. But that Wrigley Field and the NL Central would probably offset some or all of that improvement.