The referral program easily attracts more users towards the site, if it gains good trust in the short. 5% as referral bonus is really big. Some has claimed the referral bonus to be high as 50% and I don't think sites are paying high as that. At the beginning of the service paying 5% is really big and if the 5% is paid based on the wagered volume then it'll be really interesting. 
5% of the house edge is really big? No it is not, I can even say that it is the lowest referral commission in this gambling industry. As I know other sites offers at least 10% for the referral commission including the one on your signature. It is very big if it is based on the wagered amount but I believe it is 5% of the house edge.