The only efficiency graphs I have found for the Outback MX60 MPPT controller list connecting to a High(er) voltage set of arrays--not for the 12 volt nominal array:
MX60 PDF Manual (see page 33 for efficiencies)
The only efficiencies listed start with at least 12 Vdc panel voltage higher than the battery voltage (the Outback recommends a minimum of 12 volts Panel Voltage over Battery Bus voltage for the MX60).
If you read through the manual, they suggest, for example, that 6x 12Vdc (72 volt nominal array) panels in series (135vdc max working voltage, 150vdc never exceed voltage).
Except for:
The maximum input voltage for an MX60 should not exceed 150 Volts, which can happen if a 72-volt nominal array is used, in sub-zero temperatures.
For example, just a standard Kyocera KC130GT panel with:
Vmp=17.6 Vdc (@70F)
Voltage Coefficient=-0.0821 V/°C=0.14778 V/°F
Imp=7.39 Adc
Panel "Rated STC" = 130 watts
Power at 70F:
P=IV=7.39A * 17.6V = 130 watts
Power at 110F (full sun warm day, no wind, panel temperature)
P=7.39A * (17.6V - 0.14778*(110-70)) = 7.39A * 11.6888V = 86.4 watts
Power at 25F (very cold day, sun behind clouds for a while):
P=7.39A * (17.6V - 0.14778*(25-70)) = 7.39A * 24.2501V = 179.2 watts
Notice the huge amount of power difference between 25F and 110F. And at 110F panel temperature (very real in my area), you cannot even direct connect the one KC 130 panel to a 12 volt battery and float charge it (13.6V required, less than 11.7 volts from panel--neglecting voltage drop in wiring/controller).
I don't know what Siemens panels you have, or how you have them wired... But, as above, hot weather kills the output of standard Silicon based solar panels. Looking up the specs. for a Siemens 50 and 100 watt panels; the numbers look to be almost identical to the examples above.
For the 100 Watt Siemens SR100 (12V):
Vmp=17.0 Vdc (@70F)
Voltage Coefficient=-0.079 V/°C=0.1422 V/°F
Imp=5.9 Adc
Panel "Rated STC" = 100 watts
Power at 110F (full sun warm day, no wind, panel temperature)
P=5.9A * (17.0V - 0.1422*(110-70)) = 5.9A * 11.312V = 66.75 watts
Again, 11.3 volts would seem to be under the 13.6 volts required to float charge a standard lead acid storage battery at nominal temperature.
I may have found some bad data regarding the solar panel ratings above (got it from the Xantrex website--nice consolidated listing of solar panel ratings in their sizing tool)... But I don't see how any "12 volt" nominal (really around 17-18 volts Vmp rated) panel can properly charge a 12 volt battery without being placed in series behind a "MPPT" type controller in any area that gets reasonably warm (I am excluding the battery voltage change with temperature at the moment to keep things sane in this discussion).
Certainly, the panel voltages will be higher in the mornings and cooler weather--but it does not appear to be possible to charge with a standard controller and 12 volt nominal panels in hot weather with noon-day sun.
Not saying that there is anything wrong with the controller(s) or panel(s)--but you should design the entire system according to physics of the problem.
Myself, I believe in MPPT controllers installed with series connected (and/or high voltage) solar panels for exactly the above reasons -- especially if you are trying to extract the maximum amount of solar energy (and aren't we all).
-Bill