Normally, you can use up to about 75% or a little more of its capacity before you notice it cooling off. So, you were using only about 25 gallons in a shower for 10-minutes unless you have multiple heads. Not sure how many BTU's the pilot produces. My guess is it's not that many, so I'm somewhat surprised it heats things up much...maintain, I could understand with the lower losses on the newer tanks (better insulation), maybe it could heat things up, but it would be slowly.
WHen I lived in Kuwait, the storage tank on the roof got hot enough so we used the cold for hot. If where the tank is regularly gets hot during the summer, the incoming water would at least reach ambient there, and might go up a little. How hot is it in the area where the heater is located?