In the end of the day "reputation" means very little when a player comes into the MLS. TFC alone have had players who couldn't get into premiership first teams - and players like Eckersley who struggled to get into League 2 Plymouth's side - come over and played very well.
Meanwhile there are other players who are poor in lower league English teams who have done EXTREMELY well in the MLS (Giles Barnes scored 1 goal in 33 games for Doncaster, but scored 9 in 30 at Houston and looked a beast of a player when he came to BMO.
Altidore scoring only 1 in the Premiership therefore means very little. If Barnes can score 9, there is no reason why Altidore can't get mid double figure goals.
Whether that means Altidore is DP value is questionable, but I think he will be a good signing and very capable of scoring goals in this league.