which makes your system faster, having these RO pages be accessed a little faster, or having more data cached in ram so you do less disk I/O?
if your working set will fit in ram with the duplication, then it's a pretty clear (but still smallish) win, if the duplication forces pages of your working set out of ram, then it's a clear loss.
I think you may be mistaking how much of a win this is. There is an improvement in the speed of accessing memory, and even if we say that it's a 2x improvement, it still may not make much practical difference.
remember that accessing memory is already orders of magnitude slower than accessing the data in the CPU cache, so if the memory is a little slower it frequently has less of an effect than you would expect.