if he has a 2gen xb why would he go smaller all around they are 6.5". when i had my 1gen xb i put 5.25" components in the rear and had a amp on them and the 6.5 components were way stronger still, so i dont see why he would go smaller.
anything better then the paper speakers will sound better. you would also have to tune your audio to your feel.
i have a 6 year old kenwood excelon 7" headunit i still use it even if it works when it wants to but it has a badass EQ with many setting to tune my highs and bass and i am on stock speakers on my doors.