I'm not too impressed with the explorer upgrades, most of the points seem to get wasted on slightly faster exploring times, the thing is I have a whole bunch of explorers that I send at the same time, having one that is 20 minutes faster really does not help as it will still just get sent out to explore again with the others.
I would much prefer it if shorten search time was changed to more loot (ie 5% shorter time -> 5% more loot) - this way it does not make a mess of timing with multiple explorers.