You have a 15w amp, you could get away with 15w rms worth of speakers, seeing as that's not really a possibility due to I have never seen a 7.5w 12" speaker, go for 2 15w Jensen's or Weber Jensen copies,I'm not going to go into the details But you will get no advantages, not one, by using overpowered speakers in any amp, especially a low powered one,only DISADVANTAGES!
If you have the opportunity to go to a guitar shop or the like and try your amp in a Blue Dog loaded cab or anything that is 30w rms, I'm sure you won't be disappointed.Mark my words, your amp will be louder and sound better with 30w rms worth of speakers than 50.Please ignore the fact that the amps come from factory with higher rated speakers, its got nothing to do with tone,reliability,hedroom or anything other than profit for the amp manufacturers.
I'm sick of arguing the facts about this with uninformed,uneducated ignoramuses(I'm not referring to you), but if you want any more info from me on this subject feel free to PM me.I will not be replying to this thread again.