I'd like to perform the above assignment of TMR1 value as quickly as possible - as it's written it seems like 2 instructions (and probably a few more in ASM) but it seems there should be a way to assign both H/L in one shot and cut the instruction cycles in half.
No! If you read the datasheet carefully you will find;

A write to the high byte of Timer0 must also take place
through the TMR0H Buffer register. The Timer0 high
byte is updated with the contents of TMR0H when a
write occurs to TMR0L. This allows a user to write all
16 bits to both the high and low bytes of Timer0 at once.
Meaning you first write TMR0H, then TMR0L. Both register will updates at the same time you will write TMR0L.

J-P