Rain shower need good (high) water flow to rain properly otherwise you'll just get a miserable drip like prison torture.
OTOH, High water flow will not be able to heat up as well as medium water flow as the water already travel far ahead from the heating element (when its high flow).

Its part of the reason why normally they don't put rain shower as default when selling WH.
There are larger heating element WH (4800W models) if you want to run a rain shower and have it hot.

Ideally should run a new dedicated line from DB for the WH. Ask the electrician to add in 10mA RCD / RCBO (from a reputable brand ABB Hager Schneider). Contractor simply hook up only, I recommend getting a real electrician to look into this if the contractor say noneed one.. wont die one...

These don't cost that much nowadays (95 bucks for schneider with sirim sticker)
https://shopee.com.my/SCHNEIDER-RCBO-25a-2p...1048.2774414891
25A RCBO as per above can do direct replacement for MCB (if u use 4mm or 6mm cable). Else still need 20A MCB if on standard 2.5mm cable as 25A for this cable size is overload.

Why this is important?
[attachmentid=10720436]
To avoid you coming into the news like this https://www.malaymail.com/news/malaysia/202...c-shock/1896447

Standard 2.5mm cable will limit you to 3800W model WH. If you find the water too cold then nothing much you can do.
If you uprate it to 4mm or 6mm it will support 4800W (highlands model) WH.
Cost wise not that much more in terms of material for cable (even if buy loose per meter) but since pulling new line you might wanna consider just buying whole 100m rolls.

do make sure that there is a earth leakage Trip function / button on the water heater  .  and test that it works  to trip the power 
SINGAPORE, Dec 12 — An 80-year-old man and his 66-year-old wife died in their flat at Block 120 Ho Ching Road in the Jurong Lakeside district on Thursday.
The police said that they were alerted to a case of unnatural death at 4.15pm and found the couple lying motionless when they arrived at the housing block.
The pair were pronounced dead at the scene by a paramedic.
The couple’s 45-year-old son who had gone to visit them was also in the flat. He was unconscious and taken to Ng Teng Fong General Hospital, but later died in the hospital.

The police said that no foul play is suspected and investigations are ongoing.

Chinese daily Lianhe Wanbao reported that the older man Omar Manan was taking a shower when he fell in the bathroom. His wife Asmah Bujang went in to help him and both were purportedly electrocuted.

When the son Muhamad Ashikin Omar arrived at the flat with his 15-year-old daughter, he tried to save them and was reportedly electrocuted as well.
 
https://www.malaymail.com/news/singapore/20...son-als/1931245

the pana  DH  has  a 15mA  ELB  trip .  as stated in the specs. 
ELB  have been working for  many years .  and many old houses  still have these at their DB .
todays houses they use the  RCCB  in lieu. 
there is a button  on the  WH front panel  ELB Test . 
just press this and the power will trip.  mean  the  leakage test works.  do this test  once every month .
you are good to GO 

not sure about toshiba  as i only  use/ recommend  pana WH .
i only know the pana  inside and out  .