I beg to differ. Bridgestone 040's are a totally different tyre to their 050s, for example. Its not a question of being the same make its their composition, aside from the usual basics like size etc. Also, wear will have an effect. If you are finding 'zero' issues geoff-r with the mix you have on then you are limiting how far you are pushing your Zeds through the corners because you could well find that you have have a sudden loss of traction that could be expensive. I know from my two previous ZEDs that once the tread depth gets down below 4/5mm (with all four tyres the same) the grip levels signfcantly deterioate and the traction light will start kicking in - yes the ZEDs can be quite sensitive in the tyre department hence why I am banging on about not cutting corners (sorry about the pun ) when it comes to tyres.
As an anology - just look at the difference compounds in the same make of racing tyre can have. The same principles carry forward into everyday car tyres - OK not to the extremes (normally ) that racing tyres are subjected to but mixing tyre type is never going to be right - any decent manufacturer, tyre supplier will tell you that. The only explainable reason it is done is either to save money and/or that matching tyres were not available.
Demo49 - you have not said how worn are the front tyres (in fact have we seen pics of your ZED yet ) but the chances are that if the Dealer replaced the rears that says to me the fronts are likely to be at least half worn so if you are down to 4/5mm of tread that will certainly not be helping. Have you also checked the pressures (35psi all round when cold).
But I would certainly suggest that one the first 'mods' you should be thinking about are 4 good tyres that are all the same
Edit:
Ekona has just beaten me to it so what hes says I agree 100%