If you add hose, flow will go down. Now, if its on the floor, and you move it away and put it on the floor, head hight will be the same. But extra tubing will reduce flow. I added about 3 ft of tubing to a fluval 404, then measured the output from the nozzle at about 150 gph
(the manual claims 225 gph
filter circulation, 340 gal
pump output). Unfortunately, I did not measure the output before I added the hose, so I don't know how close an unaltered filter comes to the advertised output.
Now, if your fluval is right next to your tank, then doesn't that mean your tank is also right next to the baseboard heat? It would seem that if you are going to keep the filter near your tank, you should not worry about adding a few inches to the tube length.