Which GTA Online cars can you steal and own?
You can still upgrade the Sports cars like the Inferno, Banshee, etc. However, you can not upgrade them if you stole it. By LS Customs telling you the car is "too hot" it means that it's a Super Sports care that was stolen. To upgrade one of the vehicles, you have to actually spend your money on the web and purchase them. No worries though, buy insurance and a tracker for them and you will always be able to get your vehicle back even if it blows up.