does a "y" pipe cause restriction? is it bad?

been thinking about getting an exhaust for some tone. i have been on the magnaflow muffler side but i have a 325ci and it's a dual/dual 2.25'



the magnaflow i want is the magnaflow 14815, but this is a inlet/outlet2.25in./3in and singlet inlet therefore will need a "y" pipe before the muffler



my question is does a y pipe cause restriction? is it bad for the engine? lose any power?



or should i get a magnaflow 11378 which is a dual/dual 2.25 (will need to buy tip)