Moo GT

Soo since i bought the car 5 months ago and the transmission shifts kinda like a manual car ... u can feel the shifts .. a week after i bought it i had trans oil leakage and i changed the gaskets immediately and the problem went away ! i had the mechanic check the transmission after removing the torque box to change the inner gasket and he said i see no problems ( he never checked inside tho) but from it's appearence he said it's as good as new no worries it's fine ! soooo i decided to live with the transmission shifting this way shofts right , no slipping all perfect except the fact that u feel the shift went it goes into gear with each gear ! one day i used STP octane booster " a month ago" in the fuel tank that orange red bottle before i refuel it and all of a sudden my check engine light goes on and the transmission SHIFTS LIKE A CHARM ! shifting went really smooth and everything was perfect, it felt weird after the check engine got on " how on earth did things get better" then i went on the road EXCESSIVE HEAT came from the catalytic converter area from both sides " yes i used to have warmth from this area, foot area it felt weird but i read in forums that it's normal and it doesnt effect the car's performance) before using it but now it felt like it's melting" 5 mins later the feel of heat decreased back to normal warmth and the shifts are all smooth as it is ! 2 days later the check engine light went away and the shifts started to be felt kinda not smooth but still smoother than before then i thought maybe it's something in the exhaust soo i decided to remove the mufflers and have a straight pipe " shifts went kinda smoother but on full throttle WAS THE SMOOTHIEST ! full throttle now shifts like a charm all smooth going thro all gears all fine with no problems ! when i have a long distance destination and take the car moving all normal speeds 100-120KM/H on a highway for like 30 mins then go into traffic shifts get really sluggiesh and u feel every shift and then hear some soft backfire bursts from the back of the exhaust like it's stuck with something or air cant go out properly while it's down shifting ! some pop pop pop sound with no backfire tho ! " and no it's not misfiring or back firing" sparkplugs checked engine checked and everything is fine ! also after i removed the muffler the car used to shake softly when i turn it off, now no shake just turns off all smooth ! so what is that? please help ! am sick of this !