need advice on subs

[Simba#33]

ok right now i have 4 12" kicker comp subs in the back of my '98 cadillac STS (grandma car i know) with a T.M.A. 1000 watt amp. well a coworker told me yesterday after work that he was selling his 4 15" memphis subs. he has two in each box formed together to make one ported box. i was thinking about buying 2 of the 4 off of him because they come with the box and the amp wich is 1500 watts...would those 2 15's hit harder then my 4 12's? i know it may be a stupid question but im still learning this stereo stuff. thank you in advance if u do reply.