I've heard about Kakasaheb Dixit's vision of Vitthal. Were there other instances that proved Shirdi was as holy as Pandharpur?

Yes, several accounts from Chapter 4 reinforce the idea of Shirdi being a place where Lord Vitthal manifested. Beyond Kakasaheb Dixit's profound vision, which Sai Baba confirmed, there is the experience of Das Ganu. When Das Ganu was tasked with organizing a Name-week, he requested that Vitthal appear. Baba assured him that Vitthal would appear right there in Shirdi for a devotee full of faith, declaring 'This itself is Dwarka City of Ranchhod. One need not go far to see.' The text confirms that Das Ganu was indeed granted a vision of Vitthal. Another story involves devotees who, upon seeing Baba, would exclaim, 'This is the living Lord of Pandhari,' showing a widespread belief that Baba was a manifestation of Vitthal himself.