You can't amortize the expense very long without lending assistance. You can't as readily get second mortgages and low-cost money for investment property improvements, so 20-year paybacks would be impossible. It'll take some sort of gov't structured program to make it work very well, or only cash-flush small-time landlords will be likely to consider such improvements.

And no, the efficiency will not pay for itself, as the lower rents will always win regardless of efficiency. Some places premium rents go for location, but most people cannot see past the $/month for rent for potential savings on energy. They will instead convince themselves that they could use less in the cheaper place and that the numbers for the efficient place aren't "real" for their situation.