Should the XS implementation give the (overridable) isa method the precedence?

Only if you want Perl's OO to keep working. In all other cases -- every one -- Perl calls overridden methods which optionally call super methods.

(Modern versions of UNIVERSAL explicitly recommend against calling UNIVERSAL::isa directly.)

So if I understand you correctly, it's recommented to do: 
package A;

sub foo {};

package B;

sub bar {};

package C;

use vars qw(@ISA);
@ISA = qw(B);

sub foo {};
sub isa {
  return $_[1] eq 'A' or $_[0]->SUPER::isa(@_);
}

[download]
against: 
package A;

sub foo {};

package B;

sub bar {};

package C;

use vars qw(@ISA);
@ISA = qw(A B);

[download]
Or at least it's an allowed and wanted to work way? Further: It's allowed and wanted to work that: 
package C;

use vars qw(@ISA);
@ISA = qw(B);

sub foo {};
sub isa {
  return $_[1] eq 'A';
}

[download]
To prevent, C is no B?

In reply to Re^2: sv_derived_from vs. sub ISA by rehsack
in thread sv_derived_from vs. sub ISA by rehsack

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