Yes indeed, my plate for the 5.4m did say 115hp. However, my boat was ancient and it was probably 115 1970s horses (Measured at the crank and not the prop as well!)
I reckon 90hp is plenty. The 5.4m is quite a small boat for its numerical size. The hull is narrow - which is partly why they ride so well, and when I was measuring up for a trailer found that the hull is not much more than about 4.2m long.
Would not recommend putting too much weight on the transom - not because of the strength but because of freeboard. They do sit low at the stern.
It's also a light boat - about 500kg. Mine on its trailer including engine and all normal gear was less than 750kg when put on a weighbridge.
Acceleration with a 90hp on a SR5.4 is better than with a 150hp on my Scorpion.
Regarding the transom strength I think the reason that so many are reinforced by their owners is because the plywood has degraded - many are now 30 years old.
Reinforcing a transom is something that can easily (and cheaply) be done by an owner.
My transom went soft but I bit the bullet and had the old one cut out and a new one fitted - cost me £1200 though
Bolting on a few stainless bits would have been much cheaper but I did not consider that to be a "proper" and "permanent" repair that would preserve the value of the boat.
As built the transoms are fine but plywood does not last forever.
Regarding consoles if anyone is looking to do the same as Narcosis but can't be arsed with making a console, mine had an outhill console of the same type that Avon fit to the D560. Fitted fine and you can get a 100 litre tank inside it.