Electronic Cat Flaps

I know this has been discussed many times before, and I've done a
Google Groups search looking for an answer, but I'm hoping that some
very specific questions might elicit some useful extra info.

First, some background (sorry for the length). We have a Staywell 31
cat flap. This is the kind with the passive RFID type device on the
collar. There are three problems with this arrangement:

1. Sometimes the latch works fine, and Pixie (our cat) has no problem
coming in. She seems to understand that she needs to put her face
close to the flap, wait for the click, then come in. Other times it
doesn't unlock, and then she starts getting frantic, slapping at the
flap with her paw. Basically, the operating range seems to be too
short. We'd like to find a cat flap which is much more reliable in
terms of opening at the right time.

2. Pixie has a habit of sometimes slinking in quite slowly. By the
time she's got through, the latch has reset, and then the flap itself
gently glides down her tail and remains on the wrong side of the
latch. At this point, a neighbouring tabby takes advantage, comes in
and scoffs Pixie's food and sprays in the house. I've tried adding a
small weight to the flap to try and make it force its way past the
reset latch, but it seems that no amount of extra weight does the
trick. (If you think about the vector forces in operation, since the
flap is almost vertical, even a huge weight added will have virtually
no sideways component, so it's no surprise that this doesn't work).

3. Pixie is quite a small cat, and she doesn't really like the rather
large key device on her collar. She does seem to have got used to it,
but we'd prefer something smaller. A cat flap that works from an
embedded chip under the skin would be ideal, but my understanding is
that these ID chips are low-frequency devices that cannot work at long
range (ie. more than a couple of cm), so I suspect this is a
non-starter.

I see that the Staywell 31 is no longer made, and seems to have been
replaced with another model that uses an active infra-red collar key.
Does anyone have experinece with this? Is it more reliable than the
model 31? Is its collar key any smaller than the 31's big RFID type
thing? Are there any other makes which are more reliable? Cost is not
really an issue (within reason).

Thanks.