96 yukon 4x4 5.7 L. just want a little more zip...........?

can I bump up my ing. timming 4 degrees. or will it screw up my computer....also are aftermarkets computers any good on these mills ? what about T.B. spacer ? Is valve adj. zero lash then 1/2turn down....looking for simple perf. mods I can do myself...185,000 km.s still running good......any help ...TY.