Whatever the POW extender value will be, the only what matters, that IT HAS TO BE on the information sheet about the coin. Thats's it. This is the only problem.
It's a factor which miners have to know about:
block time, net hashrate, [POW block time extender]-----> difficulty, block value ---> coin price
Than will be more miners again. X4 would be good, cause than it's easy to calculate (2 min blocks). And a bit better for early miners, than X3, i guess

(actually, it corrupts net hashrate stat also, but it's not a problem:
1. calculate with the actual block time: 30 sec X extender value
or
2. calculate with lowered hashrate: net hashrate / extender value
You will get the same result, i think, so it would be clear for miners.)