There are two points to consider. First, chain is used for weight and to ensure a horizontal pull on the anchor. In a blow the warp will quickly be pulled up so that the catenary is much less - ie the warp is almost a straight line. If there is sufficient chain between the warp and the anchor then the chain will resist this effect and continue to lie horizontal or nearly horizontal along the sea bed. 15m of chain might be enough in most conditions, but maybe not if you are going to anchor in deeper water sometimes . . .
Also, warp can abrade badly on some sea bottoms.
As I understand it the main advantage of warp is that it helps prevent snatching due to its elasticity. Also, for small vessels it is lighter.
Why do you not want to use an all-chain rode? Or at least enough chain for most water depths with warp for the odd occasions when you have to let out a lot more scope in a blow or anchor in really deep water. Your boat must have plenty of room for 50m chain at least, backed up by 50m of warp.
- Nick 
