I have to run the numbers, but retargeting difficulty with a time component will create vulnerability to a TW(Time Warp) attack. Just doing a quick simulation here tells me that a TW attack would be successful with less than 10% of the total nethash.

I could very easily create a side chain by manipulating timestamps on the blocks to appear to have 150sec block times with minimal hash-rate that could be merged into the main chain at any point even if you implemented a centralised network time server because the blocks are no longer comfirmed by proof of work, but by time stamp.

As I see it, you would need very little nethash for a successful TW attack.