Crops already are invasive species. The majority of them were originally native to the Middle East and we have modified them through manual selection to grow in other regions just as successfully. We count on them to outcompete native plants (if corn (which was actually from Central America I believe) can't outproduce native prairie grasses in Iowa and Nebraska then we won't have any corn).
The point at which it could become a bad thing has already past
Crops are not invasive species. They are non-native species but that is not the same thing. An invasive species has a survival advantage over native species. Typically, this is an adaption to a threat not present outside its native environment. Crops are not like that. They are modified to produce more/better food for us. That puts them at a disadvantage against native plants (aka "weeds"). They need help to survive. That is exactly the opposite of invasive.
Direct genetic modification makes it easier to improve all crop characteristics but the basic trade-offs remain. It would certainly be possible to engineer a super tomato or super corn that would out-compete native plants and take over the landscape. It wouldn't be of any use to anyone though since the only one to get there is to make the plant nearly useless for food production.