>>but recently a software tester told me that I should code like "Query A"

A tester will no doubt find query A more 'readable'

>>you should always use upper case.

ALWAYS? FOR EVERYTHING? personally I like upper case only on RESERVED words.

>>you should put quotes in column and table names

Personal preference. It's up to you.

>>you should use the JOIN syntax because it's the recommended syntax (ISSO/IEC 9075 SQL:2003)

If it's part of that ISO standard, and you want to meet that standard, then you probably should use it.

>>And he justifies this with performance improvements.

I'd ask for proof.
As PAVB suggests, how about some numbers to back it up?

>>A) Does this make any sense?

The questions make sense.

>>B) Is the JOIN syntax better than the Oracle syntax (+) in terms of performance? Does it make any difference?

No. To readability, yes. To the ISO, probably

>>C) Does the upper case and quotes make any improvements?

No. To readability, yes for the case; for me No for the quotes.

>>D) Does any of this influences SQL statement processing phases?

Unlikely.