I am not sure that the higher GC content will give a sharper bands. There are other factors that could do the same effect such as the number of cycles and agarose concentration. Meanwhile, you can enhance you efficiency of your RAPD for revealing biodiversity  by using three good RAPD primers having the same TM conditions as descripted in the attached paper 

Conference Paper Diversity assessments among Mango (Mangifera indica L.) cult...

See also attached presentation about the Triple RAPD PCR technique. I hope this will help you

Conference Paper Triple RAPD PCR: Three primers based Random Amplification of...

Yes, Higher content of GC can give better amplification, but the most important aspect is a strict protocol. Always it is neccesary  to test several primers before a decission..

yes, ahigher concentration of CG can give better bands, but the most important aspect is a strict protocol. Always it is necessary to test several primer before a decission... 

Optimal protocol and high quality DNA are more important for successful RAPD. High GC primer can give less products of PCR from false annealing sites but it is just equal to use of optimal Ta for lower GC primer. For RAPD you should find optimal PCR conditions for each group of primers with similar GC content.  But, such protocol can give  just 3-5 bands per reaction, and many researcher are using sub-optimal conditions to get 10-15 bands of poor reproducibility.

It is not a thumbrule that in RAPD 10-mer primer with 70% GC content will always give you sharper bands, it all depends on the main experiment. Some time you do get sharp bands with primer having 60% GC content.