Toilet flushing - power

Hi - I have a "back to the wall" toilet with a concealed cistern. I have been having problems with the soil pipe blocking - apparently the flat used to have a high level cistern. The soil pipe is about 1 to 1.5 m with a very shallow fall - and a 90 degree turn halfway - before it drops away
The cistern is 6 litre capacity.

My question is, if I raise the height of the cistern will it deliver more flushing power and/or do I need to increase the volume of water being flushed out?
If I were to install a high level cistern (i.e. about 7 foot up) do i need a special toilet pan?

Any help/advice greatly appreciated
Cheers

Andy

i'd be looking at the waste pipe fall before anything.

I know where you're coming from but the waste pipe fall can't be changed - it's a old concrete building.