i'm not too sure if you realise but the inbuilt 5 point harness can only be used until your child is 18Kg anyway. Then you have to use a H harness and a seat belt. (i'm not saying anything bad about the Maxi rider when i say that either.. i have one!) The new laws do make allowances for children whoa re too tall/ heavy for the seats in their age range too. My 6yr old DS is too heavy for a booster so he is now allowed to sit on the normal seat i have even checked with the police!
Hope that helps a little bit!!