The 'inventions' done to make an antenna are generally considered a trade secret by the manufacturers. I found out how my dual band works, when it got crushed after traveling in the cargo hold of an airplane. It's not simple coils... So simulating it in nec is not a simple task.
When used with an HT, all antennas assume that the user has the antenna in his hand, and the body will act as a (bad) 'counterpoise'. Of course, if you want to improve the situation, and make the use more independent of the user's position and 'pose', a groundplane would help. If nothing else, it will reduce the RF current to the hand of the operator.
On the other hand, I'd think twice about adding 3 (or 4) pointy things to the antenna, which will mostly be traveling at or near eye-height. It seems like an invitation for lawsuits.
Note that you groundplane will only work at 70cm. If you want them to work on 2m too, you'll have to add radials for that frequency.
And a final note: if you only want 70 cm, think about looking for a J-pole based design, or a vertical dipole, Neither needs a groundplane, and move the high current node up in the antenna.