Hi zig,
As Garuf says the flow rating is optimistic at best. Instead of getting two I'd just get the next higher rated model, except Eheim really punishes you for a thermofilter pricewise. I agree with Ed that they are fiddly because of the cables and thermostat probe. Your aquascapes are extremely meticulous so you might find the probe objectionable, it is about half the size of a lead pencil but at least it's cable is very thin. You also have to find a place to mount the little digital thermostat. The calibration of thermofilters can drift over the years but the fault is more likely to be in the little thermostat or in the cable.
The external Hydor solution is certainly a good option, but despite all that I've only ever owned thermofilters and I have two of them running my tank now. I've never had one cook my fish or fail in any way except the calibration drift on one of them, and I've had a total of four. As Ed alluded to, if you have the filter mounted underneath in a cabinet, this makes life a bit awkward undoing cables and I have been know to mutter a few choice words in Eheim's direction once or twice.
Thankfully, he pro III versions have removed all these frustrations and now has everything integrated into the pump head.
Other than that the filter is very quiet and the Eheim biomedia is beyond reproach. I guess you'll be throwing the ugly spraybars and the obnoxious green Eheim garden hose tubing in the bin in favor of super-cool ADA type gear.
Cheers,