If that was the case, and it didn't boost them into impressive tanks because of a stats, then it wouldn't be the coveted magic it is. You all want to ignore the way the story has portrayed slayers, because yall want to pretend it's not a boost? So now we're going to act like SLayer mages arent the tanks of the vesre? Their not melee monsters who can take a ton of punishment. Oh ok. It's a boost when Gray gets slayer magic though. IF this was how it actaully worked, since they havne't faced a demon since tartarus. Gray's slayer magic should be irrelevant, since it's only effective agaisnt the target. Lets ignore the entire battle showing off the increased stats his slayer magic grants. Erza got no boost, her using other peoples magic is a non factor, becasue she learned everybody's magic on her own, surpassing even Wendy, her teacher, who needs an outside source to enchant something into someting.
Or Slayer magic works exactly as its been described and portrayed the entire series.