2js are not the problem the Ford GT LM race cars are... if they Slip stream and work together they can block you from taking the first place.
So you end up having to use another Ford GT to beat the fords...
Vipers don't have the grip level with out tons of tuning...
The Ford GTs may have had build problems in real life but being a game they don't in the game...
On lap ~15 ish of indy you should LAP the 2js... in the top spec Ford GT LM Spec II test car.
So no you don't need the 2J.. but you do need a full spec car and a Light car helps..
Mazda laguna seca is about acceleration not high speed.
High downforce - Lower the gears and vipers like STR ACR 08 are fine.