Regarding the idiom
If I want to emphasize how strong a particular quality of a person is, can I say:with a capital A/B etc.
said after the name of a particular quality to say that it is very strong, using its first letter
He's trouble with a capital T!
Source: Definition of with a capital A/B etc. from Cambridge Dictionary Online
He's trouble with caps! => In writing, I mean "He's TROUBLE!"
He's a jerk with caps! => In writing, I mean "He's a JERK!"
Are they intelligible in speech?
The AmE expression is "with a capital X" for example, "He's a jerk with a capital J."