Better than nothing from losing it as waste heat though. 

This table  suggests that electric motors (which underpin the compressor) can be expected to be between 80 and 90% efficiency, so there is still quite a lot to be accounted for. 

Rotary compressors usually have the electric motor in the refrigerant, as the refrigerant is used to cool the motor. Therefore the waste heat is reclaimed and becomes productive heat. 

Interesting.  COP of only 1 though for the reclaimed heat!

This table  suggests that electric motors (which underpin the compressor) can be expected to be between 80 and 90% efficiency, so there is still quite a lot to be accounted for. 

Rotary compressors usually have the electric motor in the refrigerant, as the refrigerant is used to cool the motor. Therefore the waste heat is reclaimed and becomes productive heat. 

All heat pumps, using the same gas, will have a verry simmalr perfomance, you cammot beat the rules of thermodynamics, the biggest effect on heat pump performance is the system design and how that system is opperated.

I dont doubt you are right.

That said I did some calculations a few days ago of the ratio (for a Vaillant heat pump) between the theoretical maximum COP as defined by the Carnot equation, and quoted COP.  Unless I made a mistake, which is of course possible, its close to 50%, falling to a little over 40% at higher OATs.  This table  suggests that electric motors (which underpin the compressor) can be expected to be between 80 and 90% efficiency, so there is still quite a lot to be accounted for.  I imagine some gets lost in gas leaking past the seals and doubtless there are other sources of lost energy.  However it does suggest that there is theoretically room for improvement (and variation between manufacturers), albeit that compressors are pretty mature technology so probably all perform very similarly, if indeed they aren't all made in the same factory!

the reason im asking is im thinking of possibly fitting it myself or going Octopus to keep costs within my budget.

so far ive had quotes of between £4500-7500 for just a heat pump and cylinder no rads. octopus are £3800 with 5 rad changes.

id much rather fit a Vaillant and a more efficient tank to be honest and have done all the prep work to the point where i even have 28mm primaries going to a new plant room with 13mm insulation on them, even though they are inside the house.

but rather than follow the crowd that seems to be behind Vaillant id like to check why they are using these? hence the question.

id rather start with a efficient heat pump then tweak stuff to improve my scop like larger rads as i go.

I think what really sets the brands apart is their control panels / software (and customer service/warranty), as what's in the metal box is the same for all.

I think that's the same for a lot of modern-day 'Tech'. A lot of people seem to focus on lowest price and ROI, ignoring things like fitness for purpose, useability, etc.

Heat pumps aren't new technology, so I prefer to think (maybe naively) that given they all contain similar parts, their performance should be similar. As with @cathoderay, I think far more substantial variations in performance will come from really good system design and operation. I think if you design with good system volume and flow rates, and large emitters proportionally sized to the heat loss of each room, then which metal box you install at the end of the 28mm primaries in the garden should have little overall effect.

I think what really sets the brands apart is their control panels / software (and customer service/warranty), as what's in the metal box is the same for all.

Most manufacturers publish performance data in their technical literature (for a Midea heat pump it is in the Engineering Data book) but of course this can be seen as the manufacturer marking its own homework, and furthermore the data is efficacy (tightly controlled conditions) data rather than real messy world efficiency or rather (see below) performance data.  

* I prefer to use the word performance rather than efficiency for heat pumps. Very strictly speaking, efficiency is the useful energy returned from the energy put in, which by definition cannot exceed 100% (you can't get something for nothing). A heat pump does something different, it extracts heat from the outside air, making, at least for me, the word performance seem better to describe how the heat pump, err, performs. COP also uses the P word. 

the reason im asking is im thinking of possibly fitting it myself or going Octopus to keep costs within my budget.

so far ive had quotes of between £4500-7500 for just a heat pump and cylinder no rads. octopus are £3800 with 5 rad changes.

id much rather fit a Vaillant and a more efficient tank to be honest and have done all the prep work to the point where i even have 28mm primaries going to a new plant room with 13mm insulation on them, even though they are inside the house.

but rather than follow the crowd that seems to be behind Vaillant id like to check why they are using these? hence the question.

id rather start with a efficient heat pump then tweak stuff to improve my scop like larger rads as i go.