Gun handling unmatched, except for the M60, no doubt about that. However this advantage is not good enough to outweigh all the other shortcomings. [Compared to other TX meds like E50M, Obj140, Progetto 65, STB1, …]
short distances the better gun handling hardly matters
medium distances you may need to aim at weakspots, like the lower third of the enemy tank while he can pen your whole silhouette, negating your advantage of faster aim by aiming longer
long range sniping the Leopard 1 is best in class by a 0.01 dispersion advantage, however dont forget that you mainly rely on RNG not fucking you up at 350+ meters
despite being all about the gun, DPM wise it sits right in the middle, which includes autoloaders and autoreloaders
Premium APCR round instead of HEAT: There are few situations where 323 APCR will do the trick when 278 APCR doesnt. You will always bounce at 70 degrees instead of 85 degrees and this is a HUGE drawback. It denies you many shots at well angled but thin armour plates like turret and hull roofs, side armour of side scraping tanks, ……..
70km/h is the highest top speed of all TX mediums, except for the CS63 with the turbo mode, agility wise the Leo 1 can also keep up with the rest. It can truly flex around the map for your advantage like the other meds but a bit better.
Camo and vision: To make it short, 410m view range with camo towards the lower end in the TX med ranks, you wont win the vision game against many other TX meds. UDES, Progetto, glorious heroic hover tanks of the working people, chinese meds, french meds and the CS63 will all outspot you given similar outfitting and crews. The only guys you will clearly outspot are the E50M, M48 and M60.
Armor: You have no armor, which means no staying power or free damage farming outside of using the vision game. Everytime the enemy wants to dmg you, he can easily do so without breaking a sweat. Being a quite big tank for medium standards also doesnt help you hide or work around corners outsnapping the enemy.
The Leopard 1 is a quite nice tank if you enjoy good gun handling on a fast platform. There are worse tanks like the powercrept Bat-Chat or the slow as fuck 45km/h top speed M48 Patton.
What I am trying to say is, that there is no situation where another TX med is basically as good as or flat out better than the Leopard 1.