cadillac srx 05 low compression on #6 cylinder

I am sorry to say that if it is really low on compression there is no buy it in the store fix. It will need to be fixed. If it is rings then it will be a rebuild if it is valves then a valve job is in order. There is no cheap in a can fix. Your mechanic is right those items will not fix low compression. Sorry.

the heads would include new valves. i know that there may be a problem with piytons and rings: however my direct question was,"will there be increased blow back"

i am vague on the answer. will new heads which will include the valves cause any blowback?

This all depends on the condition of the lower end. If the rings are weak to start with and a rebuild was in order and all that was done was the heads then yes it can. In other words you now have improved the compression with the heads and the valves now 100 percent seal it will put strain on the lower end. I prefer to do a compression test first then once the heads are off if there is no evidence that the valves are burned and didn't cause the low compression then I recommend a complete overhaul. If all you decide to do is the heads then you need to be easier on the engine for at least the first 500 miles or you will put stress on the lower end and blow rings. I hope this better answers your question.

great answer and thank you again.

No troubles and your welcome. If there is anything I can do let me know.