i would say, dont do just the speakers.
the stock ones (as crappy as they are) are made to be paired up with the low powered deck. adding better speakers without giving them more power will make you worse off because they are less efficient (because they are made with better, heavier materials), and often times have a response that is designed to have less bass, with better focus on other notes, because its expected that they will be paired up with a sub
the best analogy i can think of is to imagine speakers are "cars" and the deck is the motor
the 2l in your mp5 is an adequate motor for that car. but if you put the 2l in say...a bmw m5. the bmw would be worse off, then even the original mp5 was, even though the m5 is a better chassis then the mp5...it just also needs a bigger motor to make the advantages come into play.
hopefully my ramblings made some sort of sense