I have twin sidedrafts on my 4k but i think you'll find you should do other things aswell not just put the twins on
You should also have extractors exhaust and at least a cam preferably some headwork aswell to get everything flowing well
If you just put twins on you'll have to dial down the mixtures so much that it wont be worth it
However, in saying all that if you do a couple of those other things you should see a descent increase in power aswell as agood increase in responsivness. They will use more fuel but more fuel generally equals more power
And as for tuning and jetting twins, this should be done by someone who knows what they are doing as twins need to be in balance
I just spent $1000 getting mine repaired, rebuilt,balaced and tuned........And i haven't stopped smiling since
I hope that helps