1. Just to get things clear: 3 Hits with Heavy Tank is practically impossible, more then 3 is completely impossible. 2 Hits can be reliably done. So you get based on those 2 spells your ~500 less dmg with Heavy. Which is a lot at level 10 (at lvl 10 you probably won't even have more than 500 dps). Adding the extra second of stun with shortrange makes a huge difference for the target to be able to teleport away or just die before being able to*; the better speed of the Goblin (The heavy tank is just so slow. A Goblin with (partly) Riot skilled might not be the fastest, but in comparison with the Heavy it feels like a formula 1 car); a 3rd aoe damage spell (if you can afford the mana): The better ganker is pretty clearly the Goblin in my opinion. At least I am way more scared of a Goblin porting to me, than a Heavy, because I know I won't be able to use my own teleporter any more.
2. About the immunity to counterganks on earth: It is true that he has this escape, but he is a really bad ganker due to the lack of a stun. An invisibility Goblin would be a brillant ganker and could then escape easily. If he gets fed once he will raid people at cps and just run away invisible all the time, which is probably no fun to play against (breaker + detector for the whole game at all cps is also kinda expensive).
3. While I do agree that Goblin feels a bit like a "sit around doing nothing" tank until porter, I don't think that giving him such a gamebreaking change would do any good. It is better to make people use the Goblin riot spell and control those little goblins. The tank does not feel so useless while waiting for porter when you are pressuring the cp/opponent with them. At most I'd suggest reducing the mana cost of Goblin riot to encourage spending a couple of points into that. (The speed bonus is not that big without any items anyway)
*Missle range cannot make up for that. You might get the occasional long range missile kill, but you can dodge the missile and with the additional second of stun you can deal another complete dps round. The damage done until the opponent can port is thus with Goblin 500+dps+Goblin Riot (if available) higher than with Heavy. The rocket has way better utility in mid, but just for ganking it is worse.
2. About the immunity to counterganks on earth: It is true that he has this escape, but he is a really bad ganker due to the lack of a stun. An invisibility Goblin would be a brillant ganker and could then escape easily. If he gets fed once he will raid people at cps and just run away invisible all the time, which is probably no fun to play against (breaker + detector for the whole game at all cps is also kinda expensive).
3. While I do agree that Goblin feels a bit like a "sit around doing nothing" tank until porter, I don't think that giving him such a gamebreaking change would do any good. It is better to make people use the Goblin riot spell and control those little goblins. The tank does not feel so useless while waiting for porter when you are pressuring the cp/opponent with them. At most I'd suggest reducing the mana cost of Goblin riot to encourage spending a couple of points into that. (The speed bonus is not that big without any items anyway)
*Missle range cannot make up for that. You might get the occasional long range missile kill, but you can dodge the missile and with the additional second of stun you can deal another complete dps round. The damage done until the opponent can port is thus with Goblin 500+dps+Goblin Riot (if available) higher than with Heavy. The rocket has way better utility in mid, but just for ganking it is worse.