You'd have to dig in the archives for actual military documents, but you might take a look at Bashford Dean's Helmets and Body Armor in Modern Warfare (1920), which covers many types of body armor used by different nations in WWI, including some data from empirical testing, reports from the field, etc.
Quite tricky to find. The only one I've personally come across is in the Regimental History of the Lancashire Fusiliers, and describes a large raid by the 12th Battalion in April 1918 in which body armour was officially issued from Divisional stores:
"A feature of the operation was that 33 body shields per Company were issued and proved most useful in warding off bayonet thrusts and splinters of bomb and shell."
I've read references to other reports, such as a platoon-strength night raid by the 7th Bedfordshires on 27th April 1916, in which all 32 men were reportedly equipped with Dayfields and helmets.
"...Steel Helmets and Dayfield Body Shields. The former proved valuable. With regard to the latter, owing to their weight they were not very popular with the men, and it is difficult to ascertain if the men wounded were saved by the shields to any great extent" 2/Lt Harry Driver, officer commanding the raid
And the 2/7th Royal Warwicks intelligence raid on 1st October 1916:
"Steel jackets worn by all ranks was the means of saving many wounds as many men report as having been struck by pieces of shrapnel which dented the steel plate and caused a small bruise on the body"
For every passing comment praising body armour there is another bemoaning their weight and ineffectiveness, and I've not come across any objective test reports. Casualty stats are muddied by the popularity of the wide range of privately-purchased armour types that ended up at the front.
I've posted a bit more about armour in this answer, but can't help with the reports you're looking for I'm afraid.