Well, fifth-generation Alto (HA12/22) had curb weight of 630-800 KG and its facelift HA23 came with same K6A engine he installed in his Alto. And curb weight of latest Alto is in range of 610-740 KG depending on configuration.
So, the car is not that heavier in comparison. If it had been a manual then it would have zero issues till Murree but automatic does get sluggish, but still it reaches Murree without much extra effort.
And bhp gap is not that much, K6A has 54 bhp and F10A in Alto maximum bhp of 60; (doubt it, 40-60 bhp range is given on internet). Torque matters on hilly driving more though.