According to my calculations and confirmed by game testing it is possible to increase the absorption stack you build during spirit shell by using PWS, while spirit shell is active. This is very dependent on your haste.

Definitions:

H = (1+(hasterating)/4.25)*PI, where PI = 1 if you have PI up or 0 otherwise

M = %mastery from tooltip expressed as a fraction of 1

Tn = Spirit shell duration time available for casting non borrowed time spirit shell PoH after casting n PWS

Xn = total number of spirit shell PoH casts when casting n PWS

An = total absorb value of spirit shell + n PWS

BT = 1 if you have borrowed time up before activating spirit shell or 0 otherwise

PoH spirit shell absorb*5 = k

PWS absorb = k*0.42*(1+M)/(1+0.5M)

The decrease in casting time for PoH due to borrowed time is 0.15*2.5/(1.05*H), thus we can assume all casts of PoH have the same cast time and just increase the total casting by by this factor for each borrowed time proc.

Xn = int[Tn/2.5*(1.05*H)], where int is a function that returns the integer part of a real number

An = k*[Xn + n*0.42*(1+M)/(1+0.5M)]

Thus maximising the absorption buffer you can create during spirit shell requires maximising [Xn + n*0.42*(1+M)/(1+0.5M)]

This assumes that you are pre-shielding i.e. the raid is not currently taking high levels of damage, so you are just interested in the amount of absorb you can put on top of ppl in anticipation to the damage spike. Under conditions of continuous damage PWS/PoH 1:1 is automatically better HPS than straight PoH spam anyway.

Tn = 15 +(0.15*2.5)/(1.05*H)*BT - n*(1.5-0.15*2.5)/(1.05*H) = 15-(n*(1.5-0.15*2.5) - 0.15*2.5*BT)/(1.05*H)

Xn = int(6*1.05*H +0.15*BT - 0.45*n) = int(6.3*H + 0.15*BT - 0.45*n)

Xn = 6 with n = 1, requires H = 1, but that is with 0 loss of time from latency. For me I need at least 3% haste in order to get 6 spirit shell casts with zero haste so all values need to have 3% added to them to account for this.

Xn = 6 with n = 2 requires H = 1.0715

That means 7.15% unbuffed haste allows you to cast 2 extra PWS during spirit shell, which means it raises the amount of absorb by 0.9 of a full spirit shell cast at 25% mastery

Xn = 7, with n = 0 requires H = 1.0874

Xn = 6 with n = 3 requires H = 1.143

Xn = 7 with n = 1 requires H = 1.159

Xn = 7 with n = 2 requires H = 1.231

Xn = 8 with n = 0 requires H = 1.247

Thus in general there is an intermediate breakpoint for the absorb with n = 1.

With PI on

H needs to be divdied by 1.2, which means anything with H below 1.2 is baseline.

Xn = 8, n = 0 requires H = 1.039

Xn = 8, n = 1 requires H = 1.098

Hence up to 10% unbuffed haste + a latency factor (this is ~3% for me) allows you to add 7 casts of spirit shell baseline and 8, n=1 casts during spirit shell.