the answer are in ur own post bro .. as u said u spent 8k and still the issue is unresolved so it means either u are swindeled or the parts replaced are those which were already ok .. or faulty parts are replaced by faulty parts...
what is the nature of the noise ? how is it ?
what u mean by chamber ?
as far as the question of opening reduces performance i wont agree unless care is not taken during refitting..
a proper fittinng and proper use of materials and methods should absolutley cause no issue..
what is the affect of noise on bike ? ride ?
does it vibrates ?
does it eats engine oil ?
does it smokes ?
does it overheats and stops ?
whats its pick/speed ? is it normal satisfactory or drastic ?
are the valve clearance set ok ? has he double checked his settings with feeler gauge ?
is the timing ok ? may be it is altered ...