In the sharia legal system are Muslims and non-Muslims treated equally when:
placing value on their witness statements?
carrying out sentencing?
Yes and no. Sharia law is not really practiced in any country in its original form. Countries just claim for it to be the source but that's incorrect. I lived in two Middle Eastern countries and they exercise it quite differently.
As for witness statements, religon has nothing to do with it, it is more of a reputation thing. If someone is known to be righteous then his statement will be taken into account no matter what religon he follows. However, if someone, even if he's a Muslim, is known to be a liar then his statements should not be taken into consideration.
Please know that false witness statments are heavily punishable as they might lead to injustice.
Carring out sentencing also depends on situation. Non Muslims used to have a choice to be judged by a Muslim or non Muslim law in early Islamic countries. However, countries do it differently nowadays. In Qatar for example, while non Qataries can go to a night club, Qataris can't go as the law is different for them. In other countries like KSA or Iran it's their version Islamic law.
In North Africa, kids smoke weed and youngsters drink. There you can drink but you should not wander outside while drunk, otherwise you will be busted and you will do time.
Again if Islamic law is not really applied, otherwise banks would not have operated there, remember that usury is not allowed in Islam and even Islamic banks practice it in some form.
Update
I did more research concerning the subject of eyewitnessing. Schools of thought exist in Islam and different schools mean different ideas. As we progress in time, new events, traditions, habbits take place and normally schools of thoughts address those issues and decide if they are ruled as allowed or disallowed. Per example Porn and the Internet, should we forbid the internet because of pornography?
Iranians a while ago decided to start their own network, in KSA and Qatar they filtered the internet and in other country they just left it as it is and said "sinning is choice and we can not protect you from yourself".
Back to eyewitnessing, some schools refuse the eyewitnessing of non Muslims and some do. Other accept it only if it is between non Muslims
But again, Sharia law is not applied like it should be. Umar ibn Al-Khattab was the first Muslim leader after the prophet and his companion. He accepted eyewitnessing of non Muslims against Muslims:
Umar ibn Al-Khattab always favored individual freedom and self-respect. By word of mouth and through his writings he made it very clear that every human being was born free and no one should have to abase himself in front of others. Once, the son of Amr bin As abused and beat up a Copt Christian; on hearing this Umar ibn Al-Khattab had son of Amr punished publicly by the hand of the victim Copt Christian. Then Umar ibn Al-Khattab addressed both father and son and said, "Since when have you turned men into slaves, whereas they are born free of their mothers?"
Source: The 100: A Ranking Of The Most Influential Persons In History [Paperback] Michael H. Hart
It is missing that the Copt christian and his father travalled to see Umar and it was them who informed him about the injustice that took place. You want to read the whole story then look it up. Google is your best friend after all, except if you don't trust google with your data which you should not ;-)
Hope that this helped, Best regards
Sources for the internet
http://topnews.us/content/247597-iran-plans-launch-alternative-internet
http://en.wikipedia.org/wiki/Internet_in_Qatar
I would have listed other sources but I don't know the religious words in English and what I have written is from my educational background.