...recursive self-improvement doesn't in any obvious way require changing our understanding of the laws of physics.
Some people think that complexity issues are even more fundamental than the laws of physics. On what basis do people believe that recursive self-improvement would be uncontrollably fast? It is simply easy to believe because it is a vague concept and none of those people have studied the relevant math. The same isn't true for FTL phenomena because many people are aware of how unlikely that possibility is.
The same people who are very skeptical in the case of faster than light neutrinos just make up completely unfounded probability estimates about the risks associated with recursive self-improvement because it is easy to do so, because there is no evidence either way.
Throwing out a theory as powerful and successful as relativity would require very powerful evidence, and at this point the evidence doesn't fall that way at all.
On the other hand, the lower bound for GAI becoming a very serious problem is very low. Simply by dropping the price of peak human intelligence down to material and energy costs of a human (break no laws unless one hold the mind is amaterial) would result in massive social displacement that would require serious planning beforehand. I don't think it is very likely that we'd see an AI that can laugh at exp-space problems, but all it needs to be is to be too smart to be easily controlled to mess everything up.
Link: nextbigfuture.com/2011/11/faster-than-light-neutrinos-opera.html
Link: science20.com/quantum_diaries_survivor/opera_confirms_neutrinos_travel_faster_light-84763
Paper: kruel.co/paper-neutrino-velocity-JHEP.pdf
Previously on LW: lesswrong.com/lw/7rc/particles_break_lightspeed_limit/