Membranes are typically rated at 77°F and 60PSI, I read somewhere that lots of work was done in SoCal and because they have warmer water from the pipes, that's the "standard" that was applied to membranes today.
The temperature can cause some fairly large differences too, Assume you have a 100GPD filter, and 60PSI, if your water is 60°F then you'll only produce about 70GPD, if it's 70° then you'll make about 87GPD, if it's 80°F then you'll make about 105GPD.
Upper limit is somewhere in the 90's as far as damaging your membrane, so if you keep it in the 80 range you're good to go.
To increase the temperature you could buy a couple hundred feet of vinyl tubing and toss it in your sump to get warmer water, but you'll be paying for that increased production with electricity to re-warm your tank water up (however if it's a hot day and your tank is in the 'overheating" mode you could come out ahead). But the reality is unless you have something you submerge your tube into you're not going to get much out of it, even with a few hundred feet of tubing water moves too quickly to allow ambient air temperature to warm it up an appreciable amount. That said, you could throw it in a bucket of water that's sitting outside in the Sun.
However before you go getting too crazy about trying to improve the output, you should probably see where you stand as far as temperature
