There's no downside if the speakers can handle the extra input power (400W is quite a lot), and won't present an impedance that's dangerous to the amplifier in question. So the reason you get "8ohm minimum" values on most ampilfiers is because each "channel" is seeing 4 ohms - hooking up a 4 ohm or 2 ohm speaker presents the amplifier with something around 1-2 ohms (at least this is my understanding); similar to running speakers in parallel. That can be a death-blow for some amplifiers, others can do it just fine (there are some Accuphase amplifiers, for example, that will run BTL into 2 ohms just dandy (non-bridged they're spec'd down to 1 ohm); nevermind that they cost something like $40,000 a pair). Generally speaking 2 ohm speakers are quite rare though, and depending on how hard you want to ride with 4 ohm speakers, it may or may not be the end of the world; I'd be wary though (it'll probably make a lot of heat).
I also would not bridge any amplifier that doesn't explicitly say it's designed to be bridged, as it's possible to short it out. When in doubt, contact the manufacturer (not a sales drone, the manufacturer). In the case of these Parasound units, the only advantage I see for the JC1 is that it probably handles much lower impedance loads. Isn't it also one of their units that does "Auto Class A" up until a certain point? That would surely add into the cost and complexity of the device. As an aside, I think the "JC" components are also "special" for some reason (limited edition or some promotional thing, because they've got John Curl stamped all over them - it probably explains part of the price as well; I'm not saying they're a ripoff, I'm just saying the price hike is probably more to do with other features (aesthetic or otherwise) that that amplifier offers, not just the power output abilities).
Finally, I agree with what Drew said - figure out if you need the extra 3 dB (which isn't hardly anything), and if it's worth double the price. Generally speaking it isn't; if it were 10 dB that'd be another story (and as many of these threads conclude: can your speakers even survive that much power?). More sensitive speakers are probably a better idea if you need more SPLs.
Audible artefacts wise - hrm. I'd suspect if the speaker's impedance were too low you could hear some clipping right before it blew apart, but beyond that, probably not - maybe a bit of white noise up close, maybe not. Your environment's noise floor and the speakers themselves will play into that (and if you have very sensitive speakers, why do you need 400W? round and round it goes).