How much is 8 watts going to heat the water? Not much at all. Certainly not enough to be considered a LOT of heat. They have a very wide, relatively low speed flow.
Compare the specs of a seio M620 to say a via aqua 1300 powerhead.
Sieo outlet area about 12cm, flow 2400 lph power draw 8 watts.
VA1300 outlet 4cm, flow 1100 lph, power draw 15 watts.
So, flow speed from the seio is twice the volume through 3 times the area(So a lot slower) and drawing half the power.
I've filled up my sink with 10 liters of water(About all it will fit) and put my Seio M820 in it which is 3200lph. Looks like scaled up to 5 times the volume it would probably give a pretty reasonable amount of flow for current loving fish. Certainly slower flows than anything but lazy slow flowing stream. Current looks to be (in the sink) about 6-9 inches per second which is in the region of half a mile per hour. so probably a fraction of that in 5 times the volume.
I'm considering throwing it in my SA cichlid tank which at the moment already has something around 5000 lph turnover in 400L.