no down sides except you must ONLY use 91 octane+
go with dinan they remove the restrictor and the peak hp gain is at a lower rpm than turner.
They take out the mother board from passenger side firewall take the chip off and then put the new one on. Then reinstall. The hard part of installing it is that if you creat the slightest static electricity you could destroy your entire mother board and the $200 chip.
Some 318i have a soldered chip so taking it to a dealer is best even if they chrage 100 more for installation. Its better if they screw up your board because then they will have to replace it.
one more upgrade sugestion is buying stainless steel braided brake lines to make the brakes stiffer and more responsive.