The X/Y speed tables aren't for the grinder's movement on the rope, they're the speed values the grinder recieves when it falls off the end of a rope.
Instead, speed for line-guided sprites is handled in somewhat of an awkward way. Each line guide tile is actually defined as a list of X/Y offsets within the tile, and the sprite just uses a timer to step through that list. Instead of a value you can directly modify to change the speed, you can instead change how fast the sprite steps through the offset list.
There's actually some code in the disassembly already that does that. Find these lines:
INC !1534,x ;
LDA !187B,x ; if this sprite table is set...
BEQ SkipFrameChk1 ;
LDA $13 ;
BCC SkipFrameChk1 ; then the frame counter increments again every other frame
INC !1534,x ;
In the original game, this code was used to make certain lineguided sprites move faster (the grinder included) by incrementing the offset timer by an extra 1 every other frame, effectively making it move 1.5 times the normal speed.
If you want to increase the speed by an additional 1.0, add another INC !1534,x line at the top of that. If you want to reduce it by 0.5, remove the "LDA !187B,x" line through the "SkipFrameChk1:" line. Or to reduce it by 0.25, change the "LSR" line to "AND #$03" and the BCC to BNE. So, for example, here is how to do the following speeds: