I (finally) spent some time on this yesterday, and was very pleased with the results. My method so far:
Upgraded power supply to 12V 12 amp (linked earlier in the thread).
Replaced power terminals on electronics to larger screw terminal (highly recommended, no more slipping out).
Replaced bed connector on electronics to screw terminal (should be safer and more reliable).
Soldered on jumper leads as discussed in other thread.
Bought a connector compatible with stock thermistor plug and crimped/soldered it onto the silicone mat thermistor wires.
Copied and pasted thermistor table (for this thermistor) into firmware found googling and uploaded firmware
Ran bed PID autotune, recorded numbers and set the firmware with PID bed heating and these values
I am using the standard inbuilt mosfet, with the jumper leads and new terminals to prevent it gettng too hot. I googled the part number and it was rated plenty high for this application. So far it hasn't even gotten too hot to touch, even at full load. With my cooled electronics cover in place, this should be 100% reliable and safe. No solid state relay or extra wiring needed!
With the new terminals for the bed, I can swap the electrical connections for the new and old heater/thermistor in about a minute.
Leaving the thermistor setting at 6 in the firmware was very inaccurate. Firmware was reporting about 40-50 degrees BELOW what recorded IR temperature ie. 50 vs 90. Thermistor table from internet was much better, about 10 degrees off at lower temps but within a few around 100.
Band bang control with the new bed was no good. Since it heats so quickly, a 5 second cycle had it shooting above the target and then dropping below in the next cycle. Reducing the cycle to three seconds was better, but switching to PID was improved again and highly recommended.
I've yet to finalise the physical mounting of the bed, but am planning to have a thin metal plate underneath it with holes for the bed height screws, and a recess for the thicker part where the wires feed in to the silicone. I might even use the standard aluminium plate for this.
One thing I want is glass to print on, but I will get it cut 150 x 170 so that it extends over the mat 10mm at the front and back. I'm planning on printing some nice quick release clips so that I still have the full printing area but the glass panels can be swapped out easily.
I'll do a full write up with pics when it's sorted but in the mean time here is a teaser image of the heating up in Repetier. New power supply for both, standard setup for the lower line, silicone bed sitting on top of standard bed with cast iron soldering helper on top to add a bit of thermal mass. As you can see it goes from low 30s to 100 in under 2 minutes!