Recent advancements in text-to-speech (TTS) technology have revolutionised automatic speech recognition (ASR) data augmentation in low-resource settings. In particular, only a few public datasets are available for dysarthric ASR (DASR) and text-to-dysarthric-speech (TTDS) models have addressed data sparsity limitations by increasing training data samples and diversity. In this context, Grad-TTS (G-TTS) has been shown to synthesise speech with accurate dysarthric speech characteristics beneficial for DASR data augmentation; likewise, Matcha-TTS (M-TTS) has recently improved on typical speech synthesis baselines. Recent studies commonly focus on data augmentation (i.e. reference data combined with additional synthetic data). This work analyses Whisper DASR model adaptation performance using reference data and G-TTS & M-TTS generated data, and shows that comparable performance can be achieved using synthesised data only relative to reference data. Additionally, despite growing work on dysarthric data augmentation, the validation of typical TTS metrics for synthetic dysarthric data, and the development of TTDS metrics requires further research. Results of this work show that gold standard metrics for typical TTS and current dysarthric speech assessment metrics lack sensitivity to predict DASR performance and hence a phoneme posteriorgram (PPG) distance based on the Jensen-Shannon divergence (JS) as a metric for dysarthric speech synthesis is introduced, showing correlation with downstream word error rate (WER) scores.