Maxime - the HPI micro cars can be upgraded with several after market chassis kits (Ratzas went out of business years ago) but from my experience they are still not competitive against the Xray M18 in the mod micro class. The solid rear axle and the poor front diff on the MRS4 limits the cars handling.
As we have seen in local club racing at the RCO track and in stock trim with rubber tires, 5-cell battery packs and the 180 micro motors the HPI car can be competitive against the M18 with the right driver. When you add more cells, mod motors and foam tires the M18 will leave the HPI in the dust.
When you compare both of then in stock form the M18 wins hands down......i have noticed that with the mrs4 that it does need some upgrading to handle properly.......if you plan on racing recommend going with the m18.....it just needs a few hopups to handle awesome.
For racing on small tracks the M18 is way to go. Racing on large (8th scale) outdoor tracks i think the HPI on rubber tires is still a good setup for beating M18's.
Going for extreme motors the HPI is the only car because it is easy to pop in larger motors and still have a lot of room for batteries and the drivetrain can stand more than the M18. Ever seen a 12 cell setup or a 540 motor in a M18 ?