Get the car checked not just the injector mate. Plugs shouldnt melt like that if its running properly as long as u use the right ones for the spec.
It could be you have too much boost going thru and its leaning out, or a few other problems that only a tuner would be able 2 diagnose!!!
The max for it again would be down to the specific chip as some are mapable and only the person who mapped it would know so again get it to a tuners fisrt!!
Althou what plugs were in it ie the melted one?! could also be a case of the wrong plug as standard ones dont like stage 2/3 setups!!! but most are gapped to .65 mate