I have devised a metric for best use of respect points. With my current inventory, the best way to increase attack is buying Tommy guns, and the best way to increase my defense score is with combat boots.
Right now, I'm level 75, so my 375th least powerful gun is a machine pistol, with 2A, 2D. If I buy a Tommy gun (10A, 6D), it knocks one machine pistol off the list of used items by my mafia, for a net gain of 8A, 4D. That means I get an increase of 1A for every 12.5 respect points (100R/8A). Using a similar metric for melee, armor, and vehicles, I get the following:
.................A......D......R.......R/A.......R/D
Tommy........8......4......100....12.5.....25.0
Sawnoff......20.....12.....360....18.0....30.0
Columbian.....9......8......200....22.2....25.0
Serrated......10....17....400....40.0....23.5
Chainwhip....23...18.....750....32.6....41.7
C. boots.......3......4......50....16.7....12.5
Night vision..14...14....300....21.4....21.4
Full C. gear..15...25....800....53.3....32.0
Mustang......5......3....125....25.0....41.7
Yamaha......12....11....325....27.1....29.5
Thus, best way to increase attack with respect points is with Tommy guns, best way to increase defense with respect points is combat boots. Your results may vary, depending on your own inventory. This wouldn't work if I had 375 M4A1s, but as it is, I only have 7 at the moment. Try it out for yourself.