I know nothing of the Penn school, although I had a peep at its website. It looks very similar to the special school I work for doing IT support.
I think that you have to look at the national curriculum levels that your child is at. The children at the special school I work at start on the p scales in year 7 and typically leave school at the end of year 11 achieving national curriculum levels 2 or 3.
Many of the year 11s have got college courses lined up for next year. Some of then are doing courses like hair dressing, childcare, or car maintenace.
The special school I work at really looks after its kids. Everyone knows each other and its lovely the way the kids look out for each other. Also being small, the children don't get lost in the same way as a large secondary school. There is a high ratio of staff and any bullying gets stamped out quicker than mainsteam. Special schools are very sheltered and I am not sure if this is a good or a bad thing.
Children in special schools get the chance to be in the school play, sing in the school choir, play sport against other special schools, be prefects or other positions of responsiblity. This can help to build confidence.
You are right that children in special schools often don't get pushed that hard academically. The majority of kids at the special school I work at do not get to even attempt GCSE. Although some of the brighter ones are put in for GCSEs in subjects like Art, Science or Maths where language skills are less important. Since most special schools are very small it can be hard to acomodiate a child with a particular talent.
The curriculum of the school I work for is different to mainstream rather than necessarily diluted. For example they do life skills, ie learning to budget, put to gether flat pack furniture, painting and decorating, gardening, looking after animals. They also get lots of interesting outings and hands on experiences.
A couple of the children at the school I work for have part time placements. Ie. they spend a couple of days a week at a different school. Maybe this could be a way of your daughter testing out what its like to be in mainsteam and keeping her special school place if she needs it.