It's easy to know your power output when LOP. If you have an 8.5:1 compression ratio, simply multiply 14.9 x FF in gph = HP output. Other CRs have different multipliers. For example, the TC engines with 13.75:1 CRs, have a multiplier of 13.75.
ROP power output is a little more complicated to calculate and the POH numbers are usually generalizations, not actual numbers.
I can promise you that the laws of physics will insist that 70% power ROP and 70% power LOP WILL produce the same TAS. BTDT thousands of times.
As for taking longer when slower, well, sure, but the 25% fuel savings offset the 3% loss in TAS quite nicely. If you need to get there 3% faster, you'd better not accept single vector from ATC.
