It really depends on how many kills and deaths you have for example if your kd is 1 but you only have 1000 kills and 1000 deaths then it will have a big impact but if your kd is 1 but you have 1000000000 kills and 1000000000 deaths than the impact that it will have will be a lot smaller. I just made those numbers up but you should get the point that the more total kills you have the smaller impact it would have. But yes your k/d will go down some or a lot it all depends. Also the amount of deaths you get per game will be significantly lower then if you actually played and were infected because everyone is going to be camping so all the infected will rush them and not many people will be looking for you.

There is a possible chance of your xbox overheating but most of the newer xboxs have been upgraded to prevent this also I am sure that a lot of people have played for long periods of time non stop on your xbox so it is the same risk, RROD is always a risk but you can reduce the risk by installing the game to your hard drive it will greatly reduce the amount of heat produced.

I did make a comment on this about how much xp you could gain it will vary greatly but you are still earning a lot more than you would considering the fact that you are sleeping or gone. Yes if you do get infected first you will gain no points but the time limit is only 5 minutes so that is 1 game out of the many that will be played.