Speed soft cap is exactly 125 - though without a testing setup it’s basically impossible for me to tell anything past the 113/114 transition.
Testing this and some other stat info courtesy @a13o
Here is a graph about the effects of some brews, since they’re percentages not flat amounts they affect different tools differently depending on the tool/skills:
Also your movement speed is capped at 40 m/s but faster reel speeds get you to the movement cap faster, as stated by @lucadeltodecso
“Air Drag” 
the boundless engine is so thorough for the simplicity of the game. Truly not being exploited to full potential here.
Adding to your comment about rank, tool rank can be low because of the boons you want like magnet, AoE, and glow which can only add 30 - 40 rank when at target levels. AoE can add 50 but you don’t want that 
No tool with these boons will ever be “max rank” even if it’s “max forged”.
By the system of naming a tool for “T(x)” based on what it can one-shot with max skills, no tool can be forged to “T7” except some lucents. If you see someone selling “T7” tools and charging extra watch out. If you’re using mega strength a “T5” heavy tool is fine for T7. You might require a T6 balanced tool I’m not sure off hand. The difference is due to the brew buffs being a percentage again, not a fixed amount.
One reason someone will give for calling a tool “T7” is because maxing the forge boons means it’s going to require a lesser brew. But TBH if they’re charging a premium, brews are cheaper than maxing a forge.
Regarding quirks it’s worth knowing what the trade-offs are. Like one jump ahead giving you double-jump, or on icy ground giving fall armor, etc…
EDIT: It may be worth adding that sometimes buffs or buff/skill combos in Boundless are multiplicative where you might expect them to be additive. This really shows with durability drain and persisting foods, which is what originally led me to notice it.