Overfilling will often result in oil being discharged into the airbox via the crankcase ventilation system. So, you could have oil-fouled spark plugs that would cause starting and running problems.
True, but I saw no mention of excessive smoking ... the engine would produce a good deal of the dreaded ~blue smoke~ if it were burning that much oil, wouldn't it (legit question, not a dig)?
I hope youíve personally stopped by the dealer to report this in case it does result in problems. Talk directly to the manager and be pissed off but civil and tell him you want his personal assurance it will be made right if problems ensue. Avoid being told later that he never heard anything about it.
This ^ ... above all other things. Do not perform any more work to the bike until you (rightfully) file a complaint with the dealer you bought it from. Don't be wishy-washy about it, you've gone into a lot of debt (or paid out a lot of cash ... or both) and you deserve to get what you've paid for. Stand your ground.
If the bike is under warranty, which most likely it is, stop doing any work yourself until the initial PROBLEM (not maintenance) is fixed.
How are you going to prove that there was 1.5 liter to much oil in the engine?
Two things; 1.) Do no further work to the bike ... none. Take it to the dealer, express your findings so far (overfilled engine cases, hard starting, et al), and let THEM look at the exhaust pipes, sparks, and the airbox etc. 2.) Your ~word~ as a customer must be given credence. Other than that, there is no way to "prove" anything. Even if you had left the engine in an overfilled state that's still not "proof" that the dealer did it. So this notion of "proof" must be taken on the word of the customer. Keep in mind the dealer has more to lose than you do. If they're worth their salt, they'll accept responsibility for the problem.
The main thing is getting the entire ordeal documented by the dealer. Fully and in detail. It will be your only leg to stand upon should related issues pop up later on down the line.
Don't touch it anymore than you already have. Do not pull the plugs, do not open the airbox, do not wash it, do not even wipe it down. Let them make observations of the wetness of the exhaust pipe outlets, the spark plugs, the airbox, the crank vent system, et al. If there is visible evidence (such as wet exhaust pipes) point it out, make certain they note things like that in the work order.
Protect yourself, and your rather sizeable investment.
(speaking from a quarter century of owning a factory authorized warranty and repair center in two separate businesses representing over forty manufacturers).