There's no "firm" answer (yuk, yuk!). Shafts stiffness is specified by the manufacturer, so there's no objective determinant of what's stiff, firm, regular, or whatever. Typically, players with normal swing speeds will use R (Regular) shafts, while faster swingers will go for stiffer ones, but that's no guarantee, just the way to bet.
Callaway make this shaft flex, it is designed in between a regular flex and stiff flex shaft.
You can replace that shaft with any on the market. Since that shaft came with the Ping ISI Irons and had a firm flex, since the W44 were regular, you should get a stiff shaft put in for a replacement.
1. Not sure2. From Ebay buying guidehttp://pages.ebay.com/buy/guides/golf-clubs-buying-guide/Set flex vs. uniflexConsider shaft flex (the amount of bend in a shaft) when you shop. Beginners will want more flex than experienced golfersA: Geared toward seniors.Regular: Designed for average players with a 75 to 90mph swing speed.Stiff/ Firm/ Extra stiff: Designed for people with a swing speed between 90 to 110mph.Uniflex: Varying flex to fit most swing speeds.
Stiff, full, firm, solid.
hard and firm..... and stiff
Firm, stiff, not mushy
firm, solid, stiff, tough, durable etc
Limp
It's to firm and very stiff feeling
stiff, tight, rigid, close, flexed, firm, tense, unyielding
stiff, tight, rigid, close, flexed, firm, tense, unyielding
callous, consolidated, dense, firm, impenetrable, rigid, solid, stiff, tough