I think he just pulled his punches against Fat Boo to avoid a life or death battle. When he said he'd go full throttle against Kid Boo he meant that he'd use SSJ3 from the start, whereas he usually begins at base or SSJ to enjoy a good fight. SSJ3 seems extremely hard to maintain much less suppress, to the point that a fused being like Gotenks had his fusion time sapped to 5 minutes and reverted to base when charging a full power attack near the end of those 5 minutes. I don't believe a singular warrior like Goku, who literally said the transformation took much longer than the two previous due to his lack of control over the form, has the expertise to suppress the form's power. Hell it remains to be seen if even SSJ2 can be suppressed, and Goku was seen still keeping SSJ2 even after being knocked out (for a brief while).