The 90 degree trigger is not really needed for the stock M28 out of the box. Slam firing is from the loose fitting pot metal 45 deg sear set that comes with the M28. If you look at dragon64's video, mine was not as bad as his, but getting there.
Mine is so sloppy, you can even push the sear down even without pulling the trigger. The 2nd sear side slop chews up the piston over time, as you recock and fire because the 2nd sear never engages straight so that the edges of the 2nd sear start to bit into piston catch.
I have a old TM VSR10 which is using the same trigger group as a M28. Apparently only the M28 trigger group is VSR10 compatible.
The TM VSR10 also uses a plastic piston, and that has shot thousands of shots through it and no slam fire. Why? The 45 deg sear in it has no slop so it does no tear up the piston. (A new E1 M28 sear set won't solve your problem because it is sloppy new)
I have been using a real TM VSR10 trigger assembly in my M28 now for a year, and .... no slam fire. But no one sells the real TM VSR10 trigger assembly or the parts.
My suggestion is to replace the sear group with something better. I am waiting on this sear to drop in. http://arniesairsoft.co.uk/news2/47549
It is the only one I know of that has a curve 2nd sear surface to it does not chew up a plastic piston and robust looking. Best of all, it is not pot metal like the stock ones.
The M28 is great value for the money, considering it is low cost and includes a suppressor adaptor, two magazines and bipod! The sear issue is easily fixed because aftermarket stuff is finally coming.
Also consider the Ranger's new M28 piston that will be available as well. http://airsoftsniperforum.com/showthread.php?t=4951
I am sure Ranger is working on other stuff in secrecy.