I've moved this thread to the 450 sized heli section.
Yes...you can add a longer tail boom to a 450 sized helicopter.
Will it fly any better with the longer boom than a shorter, stock, boom.....I can't imagine that it would fly any better or worse, as long as the setup has been done correctly. Just as Tony said, if you have a good gyro and a good tail servo, the helicopter should fly exceptionally.
If you are interested in "stretching" a 450 sized helicopter, the only ones that I know of that sells parts to "stretch" a 450 is here:
There are loads of people that have upgraded to 480. Devil 480 FAST
If your going to do the tail, then you would be better to do the whole heli.
360mm blades, change the motor and ESC to 6S then you'll have the full 480 heli
One thing you may notice is the heli May feel mire floaty in the air. If the weight of the heli does not increase much and you put longer blades one then you will have more lift at a given head speed.
So depending on what feel you want you may want the stretch kit.
I would think you could, less rpm less lift. When setting up the heli with 380 mm blades you should be able to drop a pinion size to help reduce the head speed plus increase the torque to help with the added mass.